[cmd/fetch_repo] make cache corruption failures more clear

[tyler-french]

This failure was confusing to our users. We should make more clear error messages in cases when a GOMODCACHE might be corrupt.

Resolves #1774

Example failure:

ERROR: /home/user/go-code/WORKSPACE:69:26: fetching go_repository rule //external:com_github_bazelbuild_remote_apis: Traceback (most recent call last):
        File "/home/user/.cache/bazel/_bazel_tfrench/b97476d719d716accead0f2d5b93104f/external/bazel_gazelle/internal/go_repository.bzl", line 263, column 17, in _go_repository_impl
                fail("failed to fetch %s: %s" % (ctx.name, result.stderr))
Error in fail: failed to fetch com_github_bazelbuild_remote_apis: fetch_repo: resulting module with sum h1:xhO5v3Ac039HesaPqoVUwOy4WhRRJbmo9om245kp+AY=; expected sum h1:xhO5v3Ac039HesaPqoVUwOy4WhRRJbmo9om245kp+AY=, Please try clearing your module cache directory "/home/user/go-code/pkg/mod"
ERROR: Error computing the main repository mapping: no such package '@@com_github_bazelbuild_remote_apis//': failed to fetch com_github_bazelbuild_remote_apis: fetch_repo: resulting module with sum h1:xhO5v3Ac039HesaPqoVUwOy4WhRRJbmo9om245kp+AY=; expected sum h1:xhO5v3Ac039HesaPqoVUwOy4WhRRJbmo9om245kp+AY=, Please try clearing your module cache directory "/home/user/go-code/pkg/mod"
INFO: Build Event Protocol files produced successfully.
Computing main repo mapping: 
WARNING:Bazel failed with exit code 1