auth/file_source: don't try to read non-existent files
auth/file_source: don't try to read non-existent files

Commit b33b3e96e introduced a bug where we try to read a token file even if
it doesn't exist, causing unauthenticated run (e.g.  `gdrive about`) to fail
with the error

    Failed getting oauth client: Failed to read token: unexpected end of JSON input

Closes #257.

prasmussen/gdrive,prasmussen/gdrive