Fallback bugfixes

[julianstirling]

• Allows for both a dictionary or the configuration object to be passed to the fallback server.
• Adds a try-except block to the server lifespan to check for things that error during __enter__ as this causes an error of SystemExit(3) with no traceback. Save the caught error to the server, before re-raising.
• In the case of SystemExit(3), the fallback server extracts the caught error to give a useful traceback.

Closes #241

Before (but after #241 was fixed)

After

[rwb27]

There's a flake8 warning about some docstrings to fix, but otherwise I'm very happy to see this go in.

[barecheck]

Barecheck - Code coverage report

Total: 96.23%

Your code coverage diff: 0.06% ▴

Uncovered files and lines

File	Lines
src/labthings_fastapi/server/__init__.py	168, 329

[julianstirling]

There's a flake8 warning about some docstrings to fix, but otherwise I'm very happy to see this go in.

Fixed those, also added a bit more testing.