Minor linting of Google service module

In run(), fixes re-use of the passed-in 'text' parameter as part of the
list comprehension.
Intercept and re-raise Google Translate 503 errors

If a user accidentally falls into the Google Translate blacklist, then
Google Translate will return HTTP 503 errors to all TTS requests. When
this happens, we want to return something helpful and informative about
what is going on.

Displaying the raw HTTP 503 error to the user is confusing, and can lead
to frustration (see issue #37). This change intercepts the 503 error and
re-raises the exception with an explanation of the problem instead.