imp: return error on default branch of ack type switch statement

[crodriguezvega]

Description

The only item of #65 that still applies for ibc-go:

In OnAcknowledgementPacket, the default branch of a switch statement is used for success logic. This is bit risky to maintain if additional error codes get added in the future (in the ICS it simply says if (!ack.success)). Rather, all error and success cases for the ack.Response should be handled explicitly and the default should be an error.

closes: #65

Commit Message / Changelog Entry

type: commit message

see the guidelines for commit messages. (view raw markdown for examples)

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against the correct branch (see CONTRIBUTING.md).
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards and Go style guide.
• Wrote unit and integration tests.
• Updated relevant documentation (docs/) or specification (x/<module>/spec/).
• Added relevant godoc comments.
• Provide a commit message to be used for the changelog entry in the PR description for review.
• Re-reviewed Files changed in the Github PR explorer.
• Review Codecov Report in the comment section below once CI passes.

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (dbc11b5) 81.61% compared to head (2987201) 81.60%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5791      +/-   ##
==========================================
- Coverage   81.61%   81.60%   -0.02%     
==========================================
  Files         199      199              
  Lines       15146    15148       +2     
==========================================
  Hits        12362    12362              
- Misses       2319     2321       +2     
  Partials      465      465              

Files	Coverage Δ
modules/apps/transfer/keeper/relay.go	90.28% <60.00%> (-0.66%)	⬇️

[DimitrisJim]

looks sensible to me! do note that the Ack's ValidateBasic also guards against creation of a wacky response field.