Friendly message for invalid or unreachable repositories

[angelokurtis]

before:

message: 'failed to fetch Helm repository index: failed to cache index to temporary file: failed to fetch https://example.com/index.yaml : 404 Not Found'

now:

message: 'failed to fetch Helm repository index: looks like "https://example.com" is not a valid chart repository or cannot be reached: failed to fetch https://example.com/index.yaml : 404 Not Found'

Fixes #654