update some debug message behavior

[davidism]

• Don't intercept routing redirects if the code hasn't been changed from 308 (or 307). The message about not preserving the request method and body is not relevant for the modern status code. Also edited the message a bit, although it shouldn't be seen anymore.
• When files raises a DebugFilesKeyError, it looks like the original error rather than a chained exception.
• on_json_loading_failed tries to return the super() implementation first, then reraises any error in debug mode.

I originally started working on this due to pallets/werkzeug#2355, because I thought it might be better to not suppress JSON errors outside debug mode. 400 errors indicate a problem with client data, so it seems ok to show what that problem was. I've stopped short of doing that for this release, but want to revisit suppressing information in 400 errors later.

[davidism]

For future reference, previous tickets related to hiding 400 messages:

• Set TRAP_BAD_REQUEST_ERRORS to True as the default when debugging is enabled #382
• add BadRequestKeyError.show_exception werkzeug#1592
• Improvement/clearer key errors werkzeug#1395
• make debugging bad key errors easier #2348