Return `GIT_EINVALID` on parse errors so that direct callers of parse
functions can determine when there was a failure to parse the object.

The object parser functions will swallow this error code to prevent it
from propagating down the chain to end-users.  (`git_merge` should not
return `GIT_EINVALID` when a commit it tries to look up is not valid,
this would be too vague to be useful.)

The only public function that this affects is
`git_signature_from_buffer`, which is now documented as returning
`GIT_EINVALID` when appropriate.