Unfriendly errors when the wheel downloader can't find a wheel

[Tatskaari]

Error building target //third_party/python:_protobuf#download: exit status 1
CRITICAL:root:Couldn't find any matching urls in the index
Traceback (most recent call last):
  File "/usr/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/jpoole/proto-rules/plz-out/bin/python/tools/wheel_resolver/wheel_resolver.pex/__main__.py", line 390, in <module>
  File "/home/jpoole/proto-rules/plz-out/bin/python/tools/wheel_resolver/wheel_resolver.pex/__main__.py", line 377, in run
  File "/home/jpoole/proto-rules/plz-out/bin/python/tools/wheel_resolver/wheel_resolver.pex/__main__.py", line 367, in main
  File "/usr/lib/python3.6/runpy.py", line 208, in run_module
    return _run_code(code, {}, init_globals, run_name, mod_spec)
  File "/usr/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "tools/wheel_resolver/src/resolve.py", line 60, in <module>
  File "tools/wheel_resolver/src/resolve.py", line 51, in main
  File "tools/wheel_resolver/src/wheel_tags/tag

We should catch this and return something more useful