ENH: ExternalData: Retry download after HTTP 503 error
ENH: ExternalData: Retry download after HTTP 503 error

If a server is too busy to provide a requested object it may respond
with 503 even if it has the object.  Retry the request in this case.

Change-Id: Ifdfba14347a37133c661ce6bcfd9a2b1e1829068

CapeDrew/DCMTK-ITK,BlueBrain/ITK,BRAINSia/ITK,atsnyder/ITK,eile/ITK,biotrump/ITK,stnava/ITK,paulnovo/ITK,ajjl/ITK,LucasGandel/ITK,LucasGandel/ITK,blowekamp/ITK,PlutoniumHeart/ITK,fedral/ITK,hjmjohnson/ITK,eile/ITK,atsnyder/ITK,PlutoniumHeart/ITK,InsightSoftwareConsortium/ITK,InsightSoftwareConsortium/ITK,malaterre/ITK,GEHC-Surgery/ITK,biotrump/ITK,fbudin69500/ITK,paulnovo/ITK,stnava/ITK,zachary-williamson/ITK,spinicist/ITK,hendradarwin/ITK,fuentesdt/InsightToolkit-dev,hendradarwin/ITK,hendradarwin/ITK,fbudin69500/ITK,vfonov/ITK,LucasGandel/ITK,InsightSoftwareConsortium/ITK,heimdali/ITK,ajjl/ITK,eile/ITK,jmerkow/ITK,thewtex/ITK,LucHermitte/ITK,BlueBrain/ITK,biotrump/ITK,richardbeare/ITK,fuentesdt/InsightToolkit-dev,CapeDrew/DCMTK-ITK,LucHermitte/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,jmerkow/ITK,vfonov/ITK,vfonov/ITK,BlueBrain/ITK,rhgong/itk-with-dom,Kitware/ITK,atsnyder/ITK,atsnyder/ITK,stnava/ITK,biotrump/ITK,spinicist/ITK,spinicist/ITK,hjmjohnson/ITK,paulnovo/ITK,hinerm/ITK,fedral/ITK,PlutoniumHeart/ITK,malaterre/ITK,BlueBrain/ITK,thewtex/ITK,LucHermitte/ITK,msmolens/ITK,CapeDrew/DCMTK-ITK,ajjl/ITK,msmolens/ITK,jmerkow/ITK,BRAINSia/ITK,hjmjohnson/ITK,hinerm/ITK,richardbeare/ITK,spinicist/ITK,GEHC-Surgery/ITK,hinerm/ITK,zachary-williamson/ITK,CapeDrew/DCMTK-ITK,BRAINSia/ITK,richardbeare/ITK,spinicist/ITK,msmolens/ITK,fuentesdt/InsightToolkit-dev,rhgong/itk-with-dom,BlueBrain/ITK,LucHermitte/ITK,jcfr/ITK,rhgong/itk-with-dom,paulnovo/ITK,InsightSoftwareConsortium/ITK,blowekamp/ITK,CapeDrew/DCMTK-ITK,fedral/ITK,heimdali/ITK,paulnovo/ITK,vfonov/ITK,malaterre/ITK,eile/ITK,PlutoniumHeart/ITK,malaterre/ITK,jmerkow/ITK,LucHermitte/ITK,ajjl/ITK,msmolens/ITK,hendradarwin/ITK,LucasGandel/ITK,fuentesdt/InsightToolkit-dev,atsnyder/ITK,LucHermitte/ITK,hendradarwin/ITK,BRAINSia/ITK,atsnyder/ITK,Kitware/ITK,ajjl/ITK,fedral/ITK,PlutoniumHeart/ITK,paulnovo/ITK,malaterre/ITK,GEHC-Surgery/ITK,BlueBrain/ITK,CapeDrew/DCMTK-ITK,fuentesdt/InsightToolkit-dev,blowekamp/ITK,InsightSoftwareConsortium/ITK,LucasGandel/ITK,spinicist/ITK,stnava/ITK,thewtex/ITK,CapeDrew/DCMTK-ITK,msmolens/ITK,jcfr/ITK,GEHC-Surgery/ITK,blowekamp/ITK,jcfr/ITK,malaterre/ITK,GEHC-Surgery/ITK,fedral/ITK,fedral/ITK,eile/ITK,hjmjohnson/ITK,fbudin69500/ITK,PlutoniumHeart/ITK,msmolens/ITK,atsnyder/ITK,hinerm/ITK,fedral/ITK,hjmjohnson/ITK,eile/ITK,hendradarwin/ITK,spinicist/ITK,BRAINSia/ITK,fuentesdt/InsightToolkit-dev,thewtex/ITK,ajjl/ITK,richardbeare/ITK,jcfr/ITK,LucasGandel/ITK,fuentesdt/InsightToolkit-dev,fbudin69500/ITK,biotrump/ITK,hinerm/ITK,spinicist/ITK,msmolens/ITK,Kitware/ITK,hinerm/ITK,Kitware/ITK,heimdali/ITK,ajjl/ITK,fedral/ITK,malaterre/ITK,fuentesdt/InsightToolkit-dev,zachary-williamson/ITK,richardbeare/ITK,blowekamp/ITK,LucHermitte/ITK,paulnovo/ITK,malaterre/ITK,PlutoniumHeart/ITK,jmerkow/ITK,ajjl/ITK,CapeDrew/DCMTK-ITK,stnava/ITK,stnava/ITK,zachary-williamson/ITK,GEHC-Surgery/ITK,LucasGandel/ITK,rhgong/itk-with-dom,vfonov/ITK,hinerm/ITK,Kitware/ITK,biotrump/ITK,stnava/ITK,malaterre/ITK,vfonov/ITK,paulnovo/ITK,BlueBrain/ITK,PlutoniumHeart/ITK,heimdali/ITK,msmolens/ITK,eile/ITK,hendradarwin/ITK,fbudin69500/ITK,CapeDrew/DCMTK-ITK,hjmjohnson/ITK,zachary-williamson/ITK,zachary-williamson/ITK,rhgong/itk-with-dom,jcfr/ITK,LucasGandel/ITK,Kitware/ITK,hinerm/ITK,hendradarwin/ITK,BRAINSia/ITK,heimdali/ITK,richardbeare/ITK,jcfr/ITK,BlueBrain/ITK,rhgong/itk-with-dom,vfonov/ITK,spinicist/ITK,GEHC-Surgery/ITK,GEHC-Surgery/ITK,rhgong/itk-with-dom,hinerm/ITK,heimdali/ITK,eile/ITK,fbudin69500/ITK,blowekamp/ITK,jmerkow/ITK,jmerkow/ITK,thewtex/ITK,Kitware/ITK,fbudin69500/ITK,InsightSoftwareConsortium/ITK,fbudin69500/ITK,heimdali/ITK,zachary-williamson/ITK,stnava/ITK,biotrump/ITK,fuentesdt/InsightToolkit-dev,blowekamp/ITK,jmerkow/ITK,rhgong/itk-with-dom,eile/ITK,atsnyder/ITK,zachary-williamson/ITK,biotrump/ITK,zachary-williamson/ITK,jcfr/ITK,LucHermitte/ITK,vfonov/ITK,heimdali/ITK,thewtex/ITK,blowekamp/ITK,BRAINSia/ITK,stnava/ITK,thewtex/ITK,richardbeare/ITK,hjmjohnson/ITK,jcfr/ITK,atsnyder/ITK