output errors during updates

[jakecoffman]

When debugging issues with the CLI, if there's an error it can be frustrating as you will see a log message like ERROR <job_cli> Error during file fetching; aborting but it doesn't actually output what the error is. To capture it, you have to use -o to specify an output file and then examine the error that gets sent to the API.

With this change, it outputs the error like this:

update-job error: {dependency_file_not_found map[file-path:/missing/go.mod]}

[jeffwidman]

See also my suggestion here about adding a hint to the logs on what a user should do to see the full diff: #42 (comment)