Change validate_self_client error type to ContextError

[Farhad-Shabani]

Closes: #482

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PR author checklist:

• Added changelog entry, using unclog.
• Added tests.
• Linked to GitHub issue.
• Updated code comments and documentation (e.g., docs/).
• Tagged one reviewer who will be the one responsible for shepherding this PR.

Reviewer checklist:

• Reviewed Files changed in the GitHub PR explorer.
• Manually tested (in case integration/unit/mock tests are absent).

[codecov]

Codecov Report

Patch coverage: 17.85% and project coverage change: -0.05 ⚠️

Comparison is base (67c2c46) 71.52% compared to head (b350afb) 71.48%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #483      +/-   ##
==========================================
- Coverage   71.52%   71.48%   -0.05%     
==========================================
  Files         125      125              
  Lines       15886    15899      +13     
==========================================
+ Hits        11363    11365       +2     
- Misses       4523     4534      +11     

Impacted Files	Coverage Δ
crates/ibc/src/core/context.rs	84.34% <ø> (ø)
crates/ibc/src/hosts/tendermint.rs	0.00% <0.00%> (ø)
crates/ibc/src/mock/context.rs	73.68% <55.55%> (-0.08%)	⬇️

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[plafer]

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