Show output from gnupg if it returns unexpected error code

[joshrabinowitz]

Currently unless -v or SECRETS_VERBOSE(or SECRETS_TEST_VERBOSE in tests) is enabled, we hide all output (stderr and stdout) from gnupg when hiding files. Analysing the code history,git-secret has always hidden most of the output from gnupg (and it's only recently that the verbose options that expose the output were added).

However, we keep getting error reports about git-secret failures that are actually errors from gnupg that we suppress (especially regarding file permissions or unusable keys), and people don't see the errors until they enable verbosity.

Presumably stderr from gnupg is hidden because gnupg also has a habit of outputting non-error data to stderr, which also means we can't just hide stdout and show stderr to make sure we show errors.

I propose that we alter git-secret to always show the output (stderr and stdout) from gnupg if it unexpectedly returns a non-zero error code. This will help users understand errors, especially regarding permissions and bad keys and other issues that are currently only detected and reported by git-secret

[joshrabinowitz]

See #508 , #491, and #317 which I think this resolves.
Also related #275

[sobolevn]

I agree, that showing direct error messages is a good idea. It will eliminate a lot of confusing.