Add handling for other requests exceptions

[amstilp]

Currently the code can handle errors from the AnVIL API (in the anvil-api-calls branch) but it does not handle other exceptions from the requests module, eg ConnectionError, TimeoutError, etc.

Some stack overflow posts about this:
https://stackoverflow.com/questions/48723711/python-mock-requests-post-to-throw-exception
https://stackoverflow.com/questions/36096379/python-requests-mock-doesnt-catch-timeout-exception
https://stackoverflow.com/questions/20885841/how-do-i-simulate-connection-errors-and-request-timeouts-in-python-unit-tests

[amstilp]

Ideally this could be something resuable - a mixin or a decorator somewhere?