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.
Handling of mass generator errors with newlines

If the mass generator encounters an exception message that contains
newlines, it should convert to spaces and collapse them so that the
errors show up nicely on the dialog.
Documentation on the throttling functionality

Adds an additional word of caution about shared public IP addresses,
discouraging users from setting more aggressive throttling
configurations if they are sharing networks.
Merge branch 'chg/helpful-google-errors-37'

Brings into the develop mainline

- support for a more user-friendly intercepted error message for when
  Google Translate returning HTTP 503 (which is usually a blacklist
  kicking in),
- better summary display in the browser generator of error messages with
  newlines (such as the new user-friendly HTTP 503 message), and
- documentation addition about shared public IP addresses and the
  throttling functionality.

Closes #37.