[XLA:Python] Raise an error if no backends are successfully initialized.
[XLA:Python] Raise an error if no backends are successfully initialized.

Prior to this change, this would fail when trying to access list(backends.values())[-1], and give no indication what the actual error was.

PiperOrigin-RevId: 275283415
Change-Id: I48a4aab0aea27794dcfdc7f8f9884f8369d4c031

frreiss/tensorflow-fred,freedomtan/tensorflow,xzturn/tensorflow,karllessard/tensorflow,annarev/tensorflow,arborh/tensorflow,adit-chandra/tensorflow,Intel-tensorflow/tensorflow,Intel-tensorflow/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,tensorflow/tensorflow-pywrap_tf_optimizer,gunan/tensorflow,renyi533/tensorflow,freedomtan/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,xzturn/tensorflow,ppwwyyxx/tensorflow,cxxgtxy/tensorflow,yongtang/tensorflow,aam-at/tensorflow,xzturn/tensorflow,gunan/tensorflow,adit-chandra/tensorflow,tensorflow/tensorflow,tensorflow/tensorflow-pywrap_saved_model,gautam1858/tensorflow,jhseu/tensorflow,gunan/tensorflow,xzturn/tensorflow,frreiss/tensorflow-fred,ppwwyyxx/tensorflow,annarev/tensorflow,arborh/tensorflow,adit-chandra/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,xzturn/tensorflow,adit-chandra/tensorflow,aam-at/tensorflow,paolodedios/tensorflow,arborh/tensorflow,annarev/tensorflow,frreiss/tensorflow-fred,gautam1858/tensorflow,ppwwyyxx/tensorflow,Intel-tensorflow/tensorflow,aam-at/tensorflow,davidzchen/tensorflow,gautam1858/tensorflow,gunan/tensorflow,karllessard/tensorflow,aam-at/tensorflow,aam-at/tensorflow,tensorflow/tensorflow-pywrap_saved_model,adit-chandra/tensorflow,renyi533/tensorflow,Intel-tensorflow/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,petewarden/tensorflow,petewarden/tensorflow,karllessard/tensorflow,yongtang/tensorflow,freedomtan/tensorflow,davidzchen/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,tensorflow/tensorflow-experimental_link_static_libraries_once,gautam1858/tensorflow,Intel-tensorflow/tensorflow,ppwwyyxx/tensorflow,aam-at/tensorflow,annarev/tensorflow,ppwwyyxx/tensorflow,gautam1858/tensorflow,xzturn/tensorflow,davidzchen/tensorflow,adit-chandra/tensorflow,tensorflow/tensorflow-pywrap_saved_model,paolodedios/tensorflow,tensorflow/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,tensorflow/tensorflow,Intel-tensorflow/tensorflow,annarev/tensorflow,freedomtan/tensorflow,Intel-Corporation/tensorflow,Intel-tensorflow/tensorflow,aldian/tensorflow,arborh/tensorflow,karllessard/tensorflow,jhseu/tensorflow,sarvex/tensorflow,karllessard/tensorflow,sarvex/tensorflow,aam-at/tensorflow,renyi533/tensorflow,adit-chandra/tensorflow,aldian/tensorflow,tensorflow/tensorflow,cxxgtxy/tensorflow,Intel-Corporation/tensorflow,freedomtan/tensorflow,jhseu/tensorflow,renyi533/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,sarvex/tensorflow,annarev/tensorflow,yongtang/tensorflow,sarvex/tensorflow,frreiss/tensorflow-fred,jhseu/tensorflow,aam-at/tensorflow,tensorflow/tensorflow,Intel-tensorflow/tensorflow,ppwwyyxx/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,arborh/tensorflow,Intel-tensorflow/tensorflow,sarvex/tensorflow,tensorflow/tensorflow-pywrap_saved_model,davidzchen/tensorflow,tensorflow/tensorflow-pywrap_saved_model,gautam1858/tensorflow,yongtang/tensorflow,Intel-tensorflow/tensorflow,jhseu/tensorflow,paolodedios/tensorflow,tensorflow/tensorflow,tensorflow/tensorflow-pywrap_saved_model,xzturn/tensorflow,davidzchen/tensorflow,sarvex/tensorflow,tensorflow/tensorflow,jhseu/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,gautam1858/tensorflow,adit-chandra/tensorflow,paolodedios/tensorflow,petewarden/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,cxxgtxy/tensorflow,arborh/tensorflow,tensorflow/tensorflow-pywrap_saved_model,renyi533/tensorflow,gunan/tensorflow,karllessard/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,aam-at/tensorflow,gautam1858/tensorflow,yongtang/tensorflow,cxxgtxy/tensorflow,karllessard/tensorflow,aldian/tensorflow,tensorflow/tensorflow,Intel-Corporation/tensorflow,arborh/tensorflow,ppwwyyxx/tensorflow,yongtang/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,aldian/tensorflow,karllessard/tensorflow,paolodedios/tensorflow,adit-chandra/tensorflow,aldian/tensorflow,arborh/tensorflow,frreiss/tensorflow-fred,cxxgtxy/tensorflow,adit-chandra/tensorflow,freedomtan/tensorflow,xzturn/tensorflow,petewarden/tensorflow,davidzchen/tensorflow,Intel-Corporation/tensorflow,gunan/tensorflow,xzturn/tensorflow,jhseu/tensorflow,davidzchen/tensorflow,yongtang/tensorflow,sarvex/tensorflow,ppwwyyxx/tensorflow,adit-chandra/tensorflow,tensorflow/tensorflow-pywrap_saved_model,annarev/tensorflow,tensorflow/tensorflow,Intel-Corporation/tensorflow,yongtang/tensorflow,gunan/tensorflow,Intel-Corporation/tensorflow,petewarden/tensorflow,davidzchen/tensorflow,ppwwyyxx/tensorflow,petewarden/tensorflow,gunan/tensorflow,ppwwyyxx/tensorflow,freedomtan/tensorflow,freedomtan/tensorflow,xzturn/tensorflow,renyi533/tensorflow,gautam1858/tensorflow,arborh/tensorflow,xzturn/tensorflow,ppwwyyxx/tensorflow,karllessard/tensorflow,frreiss/tensorflow-fred,tensorflow/tensorflow-pywrap_tf_optimizer,tensorflow/tensorflow-pywrap_tf_optimizer,renyi533/tensorflow,paolodedios/tensorflow,renyi533/tensorflow,gautam1858/tensorflow,jhseu/tensorflow,arborh/tensor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