fix: make plugin more verbose

[dandhlee]

When folks are debugging it's hard to see which part of the docstring there's an error, if they're given only a short snippet. To help track down where the issue originates from, while the original docstring is lost, I've tried to include as much information as possible to help track down where the broken docstring lives in.

Example of additional output with the updated format:

ValueError: In google/cloud/bigquery/job/copy_.py
For module google.cloud.bigquery.job, type method:
Failed to parse docstring on google.cloud.bigquery.job.CopyJob.from_api_repr.

Fixes #122.

CC @jimfulton

[dandhlee]

string append using += can be quadratic runtime, where as string append using an array and "".join(string_array) is linear run time. 😮

See https://google.github.io/styleguide/pyguide.html#310-strings

Will include this to part of #118.