Improve MultipleFailureException message to include first cause

[ascopes]

This further improves error reporting to provide more details out of the box when users report issues.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.76%. Comparing base (7ec957e) to head (30da9bd).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #426      +/-   ##
==========================================
+ Coverage   87.71%   87.76%   +0.06%     
==========================================
  Files          33       33              
  Lines        1122     1127       +5     
  Branches       75       76       +1     
==========================================
+ Hits          984      989       +5     
  Misses        104      104              
  Partials       34       34              

Files with missing lines	Coverage Δ
...ufmavenplugin/utils/MultipleFailuresException.java	100.00% <100.00%> (ø)