Update GCP Batch plugin to return stdout, stderr, and runtime exceptions

[araghukas]

See related issue: AgnostiqHQ/covalent-awsbatch-plugin#53

Currently, the GCP Batch plugin uses a google.cloud.storage.Client to retrieve only the task result and any runtime exceptions. However, the stdout/stderr output streams are neglected.

There are two important reasons for fixing this:

1. The intended UX is incomplete without task outputs being relayed back to the user.
2. The present QElectron DB implementation extracts the DB-relevant data from the output stream. If the task output is inaccessible, there will be no server-side QElectron DB and hence no QElectron UI functionality.