fix: add S2N_ERR_ILLEGAL_PARAMETER error and alert mapping

[akshatsinha0]

Introduce a new error code S2N_ERR_ILLEGAL_PARAMETER to distinguish between unexpected message types (S2N_ERR_BAD_MESSAGE) and invalid message content. This improves TLS alert compliance by mapping the new error to S2N_TLS_ALERT_ILLEGAL_PARAMETER.

Changes:
add S2N_ERR_ILLEGAL_PARAMETER to error enum in s2n_errno.h,add error description in s2n_errno.c,map S2N_ERR_ILLEGAL_PARAMETER to S2N_TLS_ALERT_ILLEGAL_PARAMETER
This resolves the TODO comment about incomplete alert mappings

Release Summary:

Introduces a new error code S2N_ERR_ILLEGAL_PARAMETER for TLS alert compliance by distinguishing b/w unexpected message types and invalid message content in accordance with RFC specifications

Resolved issues:

Partially addresses the TODO comments in tls/s2n_alerts.c regarding incomplete error-to-alert mappings.

Description of changes:

Describe s2n’s current behavior and how your code changes that behavior. If there are no issues this PR is resolving, explain why this change is necessary.

Call-outs:

Address any potentially confusing code. Is there code added that needs to be cleaned up later? Is there code that is missing because it’s still in development? If a callout is specific to a section of code, it might make more sense to leave a comment on your own PR file diff.

Testing:

Code compiles without syntax errors, verified error enum ordering is correct (added after S2N_ERR_BAD_MESSAGE in S2N_ERR_T_PROTO category)

Remember:

• Any change to the library source code should at least include unit tests.
• Any change to the core stuffer or blob methods should include CBMC proofs.
• Any change to the CI or tests should:
  1. prove that the test succeeds for good input
  2. prove that the test fails for bad input (eg, a test for memory leaks fails when a memory leak is committed)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[maddeleine]

This is incomplete. The problem is not only that we don't have an error for illegal parameter. It's that sometimes we emit S2N_ERR_BAD_MESSAGE in places that actually indicate S2N_TLS_ALERT_ILLEGAL_PARAMETER instead of S2N_TLS_ALERT_UNEXPECTED_MESSAGE. So the complete fix would be to go through our code and swap out S2N_ERR_BAD_MESSAGE for S2N_ERR_ILLEGAL_PARAMETER where it is more appropriate.