Improve missing field error messages

[AllanChain]

Missing required fields currently surface as a bare SerdeError("'field'"). For example,

import serde


@serde.serde()
class A:
    a: int


print(serde.from_dict(A, {}))

The above code raises the following error before the fix:

serde.compat.SerdeError: 'a'

Here we catch that KeyError at the required-field lookup site, so pyserde raises a contextual SerdeError with the field and class name while leaving defaulted-field fallback behavior unchanged.

After the fix, the message becomes:

serde.compat.SerdeError: missing required field 'a' while deserializing A

---

The fix was done with the assistance of LLM, and I reviewed and tested it. It looks reasonable, but I'm not certain if the fix is ideal.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.82%. Comparing base (f09d503) to head (0c0490e).
⚠️ Report is 69 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #747      +/-   ##
==========================================
- Coverage   90.83%   90.82%   -0.02%     
==========================================
  Files          13       13              
  Lines        2040     2310     +270     
  Branches      370      451      +81     
==========================================
+ Hits         1853     2098     +245     
- Misses        124      141      +17     
- Partials       63       71       +8     

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