Extension throwing an exception provides no helpful debug info

[dstansby]

Describe the bug
When an extension throws an exception, sphinx errors and halts, but provides no information on what the exception was or how to debug it further. It would be helpful if more information on the exception thrown could be shown, or the path to a log that can be further used to debug was shown.

To Reproduce
Steps to reproduce the behavior:

$ make html -d
GNU Make 3.81
Copyright (C) 2006  Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.

This program built for i386-apple-darwin11.3.0
Reading makefiles...
Reading makefile `Makefile'...
Updating makefiles....
 Considering target file `Makefile'.
  File `Makefile' does not exist.
  Finished prerequisites of target file `Makefile'.
 Must remake target `Makefile'.
 Successfully remade target file `Makefile'.
Updating goal targets....
Considering target file `html'.
 File `html' does not exist.
 Looking for an implicit rule for `html'.
 Trying pattern rule with stem `html'.
 Trying rule prerequisite `Makefile'.
 Found an implicit rule for `html'.
  Pruning file `Makefile'.
 Finished prerequisites of target file `html'.
Must remake target `html'.
Putting child 0x7f9accc05790 (html) PID 51850 on the chain.
Live child 0x7f9accc05790 (html) PID 51850 
Running Sphinx v3.1.1
loading intersphinx inventory from https://docs.python.org/objects.inv...
intersphinx inventory has moved: https://docs.python.org/objects.inv -> https://docs.python.org/3/objects.inv

Extension error:
Handler <function process_automodsumm_generation at 0x7fcacc1d1160> for event 'builder-inited' threw an exception
Reaping losing child 0x7f9accc05790 PID 51850 
make: *** [html] Error 2
Removing child 0x7f9accc05790 PID 51850 from chain.

Expected behavior
More information on the exception thrown by the extension to be provided, so it can be debugged.

Your project
https://github.com/dstansby/soloswapy/

Environment info

• OS: [e.g. Unix/Linux/Mac/Win/other with version]: macos 10.15.5
• Python version: [e.g. 3.7.1] 3.8.3
• Sphinx version: [e.g. 1.8.2] 3.1.1
• Sphinx extensions: [e.g. sphinx.ext.autodoc, recommonmark]
• Extra tools: [e.g. Browser, tex or something else]