Better error handling for unblock_user method

[caiosba]

Description

When trying to unblock a tipline user, don't crash if there is no related instance in the blocked_tipline_users table.

How has this been tested?

Existing tests should cover this.

Checklist

• I have performed a self-review of my own code
• I have added unit and feature tests, if the PR implements a new feature or otherwise would benefit from additional testing
• I have added regression tests, if the PR fixes a bug
• I have added logging, exception reporting, and custom tracing with any additional information required for debugging
• I considered secure coding practices when writing this code. Any security concerns are noted above.
• I have commented my code in hard-to-understand areas, if any
• I have made needed changes to the README
• My changes generate no new warnings
• If I added a third party module, I included a rationale for doing so and followed our current guidelines

[codeclimate]

Code Climate has analyzed commit 6ba2270 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (100% is the threshold).

This pull request will bring the total coverage in the repository to 100.0% (0.0% change).

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