This makes it easier to determine what exactly went wrong and thus build
better error handling in a later commit.
Tests were adapted accordingly to assert that only those specific errors
are raised instead of the plain ValueErrors.

Previously and ImproperlyConfigured error was always raised when an invalid
default configured for the BooleanValue class.
This behavior is unnecessary if the default is never used because the
value is marked as environ_required.
The check has thus been adapted to ignore invalid defaults in that case.

This introduces a function that wraps django entry points with an error
handler so that our own exception can be pretty-printed to an end-user
without a large stack-trace.
The defined error handler has also been applied to the existing
management, wsgi, asgi and fastcgi entry point definitions.

This way, multiple ConfigurationErrors are caught during setup, accumulated and printed all at once

Now Value classes accept a 'help_text' keyword which will get printed when a
ValueRetrievalError or ValueProcessingError occurs

Now Value classes accept a 'help_reference' keyword which will get printed when a
ValueRetrievalError or ValueProcessingError occurs

See #332

The test failed for two reasons. One was me changing the error string in a previous commit which
I have adapted the test for now.
The other was that django-cache-url changed its behavior. See #332

Example generators are an addition to the 'Value' classes that allow the error handler to display
example values for the failed value.
For example if no django secret key is found to be defined in the environment, an example (secure)
value for such a secret key can now be generated on the fly and be suggested to the user