Failure of loading drawings will now close the drawing view and show an
error message, hinting at file or permission issues, instead of leaving
the user facing a continuosly loading interface.
Adds test to cover.
This also updates errors from our HTTP service to be wrapped in a custom
error type for better identification and so the error is an actual
javascript error. Should be object compatible.
Related to #3955.
- Allowed DRAWIO env option to be passed as URL to point to instance.
- Updated tests to check URL gets passed to pages correctly.
- Update default URL to be the default theme.
For #826
