Improve gas messages

[grarco]

Describe your changes

Improves the gas error messages we emit from mempool validation and process proposal.
Adds two new unit tests.

Checklist before merging

• If this PR has some consensus breaking changes, I added the corresponding breaking:: labels
  • This will require 2 reviewers to approve the changes
• If this PR requires changes to the docs or specs, a corresponding PR is opened in the namada-docs repo
  • Relevant PR if applies:
• If this PR affects services such as namada-indexer or namada-masp-indexer, a corresponding PR is opened in that repo
  • Relevant PR if applies:

[codecov]

Codecov Report

Attention: Patch coverage is 99.05660% with 1 line in your changes missing coverage. Please review.

Project coverage is 73.41%. Comparing base (051ec84) to head (9e807be).
Report is 21 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/node/src/shell/process_proposal.rs	98.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3928      +/-   ##
==========================================
+ Coverage   73.34%   73.41%   +0.06%     
==========================================
  Files         341      341              
  Lines      105522   105670     +148     
==========================================
+ Hits        77400    77577     +177     
+ Misses      28122    28093      -29     

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