Make coverage-recording errors non-fatal

[Porges]

When we record coverage, if any of the files fail (e.g. due to timeouts during coverage recording), then we fail the whole process.

Instead, make coverage recording best-effort: if any files fail then we continue to record coverage using the other files. A warning is printed that we can monitor for any ongoing problems.

Closes #3041

[codecov-commenter]

Codecov Report

Merging #3166 (7280cc1) into main (37aede8) will increase coverage by 0.30%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #3166      +/-   ##
==========================================
+ Coverage   29.61%   29.91%   +0.30%     
==========================================
  Files         332      301      -31     
  Lines       38885    33261    -5624     
==========================================
- Hits        11516     9951    -1565     
+ Misses      27369    23310    -4059     

Impacted Files	Coverage Δ
...c/agent/onefuzz-task/src/tasks/coverage/generic.rs	0.00% <0.00%> (ø)
src/agent/onefuzz-telemetry/src/lib.rs	2.62% <0.00%> (-0.01%)	⬇️

... and 53 files with indirect coverage changes

[Porges]

We should really generate an event here.