refactor(triangle): raise if output files not found

[wpbonelli]

• previously Triangle.build() would continue silently if expected output files not found after invoking exe
• raise FileNotFoundError immediately instead
• remove unneeded return statements
• motivation

[codecov]

Codecov Report

Merging #1954 (6cf1c5b) into develop (9e9d730) will decrease coverage by 0.7%.
Report is 1 commits behind head on develop.
The diff coverage is 100.0%.

@@            Coverage Diff            @@
##           develop   #1954     +/-   ##
=========================================
- Coverage     72.6%   72.0%   -0.7%     
=========================================
  Files          257     257             
  Lines        57812   57653    -159     
=========================================
- Hits         42009   41537    -472     
- Misses       15803   16116    +313     

Files Changed	Coverage Δ
flopy/utils/triangle.py	65.7% <100.0%> (-0.2%)	⬇️

... and 72 files with indirect coverage changes