Document SECRET_KEY becoming required in 1.5.

  Also expand the description slightly, and add a more prominent warning
  about the security implications of running with an exposed SECRET_KEY.

For backwards compatibility with stupid people like me. Refs #18772.

By using settings config, we take advantage of custom settings like
KEY_PREFIX which allows for example to run tests in parallel.

Removed several inappropriate .encode('utf-8') calls in the process.

In Python 3, \w matches any Unicode character.

Thanks thecore for the report.

Complementary to commit 62a9ed0a.

In some cases (notably Python 3), when handle_uncaught_exception was
itself raising an exception, the got_request_exception was storing
the latter exception instead of the original exception.