fix: allow subject/procedures 400 responses, expose error messages, fix typo

[dbirman]

This PR fixes two issues that were flagged by users:

• The subject/procedures endpoints respond with 400 for invalid responses, but the page was only accepting 200. It now accepts 200 + 400 responses but shows the 400 ones in a yellow box to highlight that they are invalid
• When a non 200/400 response returns the page now exposes the actual error. So it shows "Not found" when for example a subject doesn't exist. Previously it just said there was an error.
• While fixing these I noticed some small typos: the procedures section still claims we don't merge, but we do merge now, and there was a missing "to" in a sentence early on

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📚 Documentation preview 📚: https://scicomp-docs--159.org.readthedocs.build/en/159/