Simplify error handling and restructure runner.rs

[Robbepop]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 77.64706% with 19 lines in your changes missing coverage. Please review.

Project coverage is 81.35%. Comparing base (25a613f) to head (7ad6ba3).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/wasmi/tests/spec/runner.rs	77.64%	19 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1278   +/-   ##
=======================================
  Coverage   81.35%   81.35%           
=======================================
  Files         306      306           
  Lines       25251    25248    -3     
=======================================
- Hits        20543    20541    -2     
+ Misses       4708     4707    -1     

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[Robbepop]

The Miri (spec) job experienced a time-out despite the recent increase in time.