Flatten PKI error variants

[djc]

This gets rid of the WebPkiError and WebPkiOp types in favor of a small number of Error variants:

• InvalidCertificateEncoding -- represents BadDer and BadDerTime
• InvalidCertificateSignature -- represents InvalidSignatureForPublicKey
• InvalidCertificateSignatureType -- represents UnsupportedSignatureAlgorithm and UnsupportedSignatureAlgorithmForPublicKey
• InvalidCertificateData

This plays off of the distinctions useful to the bogo test suite as well as the need to send a different alert for invalid encoding errors.

Alternatively, we could still combine this with WebPkiOp. However, my feeling is that WebPkiOp doesn't quite carry its weight.

Fixes #782.

[codecov-commenter]

Codecov Report

Merging #792 (432ed5e) into main (b84721e) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #792      +/-   ##
==========================================
- Coverage   96.53%   96.52%   -0.01%     
==========================================
  Files          57       57              
  Lines        9230     9104     -126     
==========================================
- Hits         8910     8788     -122     
+ Misses        320      316       -4     

Impacted Files	Coverage Δ
rustls/src/lib.rs	100.00% <ø> (ø)
rustls/src/client/hs.rs	97.63% <100.00%> (ø)
rustls/src/error.rs	98.01% <100.00%> (-0.22%)	⬇️
rustls/src/verify.rs	74.88% <100.00%> (+0.94%)	⬆️

---

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 b84721e...432ed5e. Read the comment docs.