Provide ability to test failing issue report submission in mock environment

Background

In #55 (closed) we made the issue reporter compatible with our test server so that we can go through the entire process without having to spam our servers with dummy issue reports. However, we still cannot reproduce scenarios in which the server returns an error.

What to change

For issue reporter:

• Provide ability to specify whether the issue report submission should fail.
• Provide ability to specify error that should be returned by the server.