#4923 Fix/trade monitor freezing

A description of the changes proposed in the pull request: The issue is due to various exchanges sporadically failing to fetch the last traded prices which causes the loop to fail entirely. The solution is simply to log the exception and continue looping. Addressing the individual exchanges' sporadic issues should happen in separate tasks.

Tests performed by the developer:

Adapts the existing test cases and adds new one for the fix. Tips for QA testing: Same testing procedure as performed so far.

PR Link: https://github.com/hummingbot/hummingbot/pull/4923