Update response model information for routes

[CasperWA]

Use a single pydantic model for all routes' response_model and add a
dictionary of erroneous status codes with the ErrorResponse model to
all routes' responses property, updating the OpenAPI schemas with the
correct mapping of response models to status codes.

Fixes #763

[CasperWA]

This could be improved further, I think, where the specific errors (e.g., 553 Version Not Supported) is explicitly named as such with a title or description in the OpenAPI schema?

[codecov]

Codecov Report

Merging #846 (9054d09) into master (c6f9977) will decrease coverage by 0.05%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master     #846      +/-   ##
==========================================
- Coverage   92.70%   92.64%   -0.06%     
==========================================
  Files          67       67              
  Lines        3699     3724      +25     
==========================================
+ Hits         3429     3450      +21     
- Misses        270      274       +4     

Flag	Coverage Δ
project	92.64% <91.66%> (-0.06%)	⬇️
validator	92.64% <91.66%> (-0.06%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
optimade/server/exceptions.py	90.24% <86.66%> (-9.76%)	⬇️
optimade/server/routers/index_info.py	100.00% <100.00%> (ø)
optimade/server/routers/info.py	95.45% <100.00%> (-0.20%)	⬇️
optimade/server/routers/links.py	100.00% <100.00%> (ø)
optimade/server/routers/references.py	100.00% <100.00%> (ø)
optimade/server/routers/structures.py	100.00% <100.00%> (ø)
optimade/server/routers/versions.py	100.00% <100.00%> (ø)
optimade/server/schemas.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c6f9977...9054d09. Read the comment docs.

[CasperWA]

This could be improved further, I think, where the specific errors (e.g., 553 Version Not Supported) is explicitly named as such with a title or description in the OpenAPI schema?

Hereby done.
I also removed these extra responses for /versions as I don't believe they apply here?

[ml-evs]

Thank you @CasperWA for your hard work

[CasperWA]

Thank you @CasperWA for your hard work

Woot woot.

Thank you!

Rebased this, keeping a commit for each of us and then force merging it, since the content didn't change since your approval.