Improve error message when cred helper not found (#55)
Improve error message when cred helper not found (#55)

* Improve error message when cred helper not found

This makes problems like
https://github.com/bazelbuild/rules_docker/issues/196 easier to debug.
For me it happened because `puller` was run by Bazel with a restricted
PATH that didn't include `docker-credentials-gcr`. The new error in this
case is:

```
Traceback (most recent call last):
  File "/usr/local/google/home/rodrigoq/.cache/bazel/_bazel_rodrigoq/a2995359558cbf75786e44ea10657e9b/execroot/containerregistry/bazel-out/k8-fastbuild/bin/puller.runfiles/containerregistry/tools/fast_puller_.py", line 97, in <module>
    main()
  File "/usr/local/google/home/rodrigoq/.cache/bazel/_bazel_rodrigoq/a2995359558cbf75786e44ea10657e9b/execroot/containerregistry/bazel-out/k8-fastbuild/bin/puller.runfiles/containerregistry/tools/fast_puller_.py", line 83, in main
    with v2_2_image.FromRegistry(name, creds, transport, accept) as v2_2_img:
  File "/usr/local/google/home/rodrigoq/.cache/bazel/_bazel_rodrigoq/a2995359558cbf75786e44ea10657e9b/execroot/containerregistry/bazel-out/k8-fastbuild/bin/puller.runfiles/containerregistry/client/v2_2/docker_image_.py", line 364, in __enter__
    self._name, self._creds, self._original_transport, docker_http.PULL)
  File "/usr/local/google/home/rodrigoq/.cache/bazel/_bazel_rodrigoq/a2995359558cbf75786e44ea10657e9b/execroot/containerregistry/bazel-out/k8-fastbuild/bin/puller.runfiles/containerregistry/client/v2_2/docker_http_.py", line 198, in __init__
    self._Refresh()
  File "/usr/local/google/home/rodrigoq/.cache/bazel/_bazel_rodrigoq/a2995359558cbf75786e44ea10657e9b/execroot/containerregistry/bazel-out/k8-fastbuild/bin/puller.runfiles/containerregistry/client/v2_2/docker_http_.py", line 285, in _Refresh
    'Authorization': self._basic_creds.Get()
  File "/usr/local/google/home/rodrigoq/.cache/bazel/_bazel_rodrigoq/a2995359558cbf75786e44ea10657e9b/execroot/containerregistry/bazel-out/k8-fastbuild/bin/puller.runfiles/containerregistry/client/docker_creds_.py", line 160, in Get
    raise Exception("executable not found: %s" % bin_name)
Exception: executable not found: docker-credential-gcr
```

google/containerregistry,google/containerregistry