Capture taxjar timeout exceptions in exception handler

[Noah-Silvera]

In some situations when Taxjar is down, a Rack::Timeout::RequestTimeoutException is thrown.

When this occurs, the exception handler does not catch this error, as it does not inherit from standard errors.

We can be reasonably confident that if this exception is thrown from a line which calls the taxjar api, that the taxjar API is unresponsive / down.

Therefore, we should catch this exception in API calls and delegate it to the configured exception handler.

This ticket arose out of the discussion in #60