Log import failure tracebacks during backend autodetection fallback.

This should help troubleshooting backend autodetection issues.

Author: anntzer

lib/matplotlib/pyplot.py

@@ -429,6 +429,8 @@ def switch_backend(newbackend: str) -> None:
             try:
                 switch_backend(candidate)
             except ImportError:
+                _log.debug("Skipping backend candidate %r as loading failed.",
+                           candidate, exc_info=True)
                 continue
             else:
                 rcParamsOrig['backend'] = candidate