This change adds basic logging of tool executions (more work should be done

later on determining when a tool execution has timed out, etc.). Review Bot

workers will now update their corresponding tool execution with their status

whenever there is a failure or when they have just begun 'running'.
For an error, the tool execution will be updated with the status 'failed' and a

result containing the error message.

Manually executed a tool profile with errors, and saw a PUT with with the

'failed' status and the error message. Verified that the tool execution object

had the correct contents and that the tool profile was still executable. A

review was not published.
Manually executed a tool profile that had no errors, and saw a PUT that updated

the tool execution's status to 'running', followed by a PUT with the

'succeeded' status and JSON review results. A review was published.