[Relay] Improve build error when no lowered funcs are produced (#4132)
[Relay] Improve build error when no lowered funcs are produced (#4132)

* Improve build error when no lowered funcs

* Switch from fatal to warning

Laurawly/tvm-1,dmlc/tvm,sxjscience/tvm,dmlc/tvm,dmlc/tvm,sxjscience/tvm,dmlc/tvm,Laurawly/tvm-1,tqchen/tvm,Laurawly/tvm-1,dmlc/tvm,sxjscience/tvm,tqchen/tvm,dmlc/tvm,dmlc/tvm,sxjscience/tvm,Laurawly/tvm-1,sxjscience/tvm,Laurawly/tvm-1,Laurawly/tvm-1,Laurawly/tvm-1,tqchen/tvm,tqchen/tvm,tqchen/tvm,sxjscience/tvm,dmlc/tvm,Laurawly/tvm-1,Laurawly/tvm-1,sxjscience/tvm,tqchen/tvm,Laurawly/tvm-1,sxjscience/tvm,tqchen/tvm,tqchen/tvm,tqchen/tvm,dmlc/tvm,tqchen/tvm,sxjscience/tvm