Fix a bug in the check for divergence in _fit_2D_poly

[mcara]

prev_max_error was not updated ever and if max_resid > prev_max_error: was always False. This effectively turned off checking for divergence.

[codecov]

Codecov Report

Merging #414 (8bcce47) into master (e15e4d4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #414   +/-   ##
=======================================
  Coverage   86.82%   86.83%           
=======================================
  Files          23       23           
  Lines        3758     3759    +1     
=======================================
+ Hits         3263     3264    +1     
  Misses        495      495           

Impacted Files	Coverage Δ
gwcs/wcs.py	87.80% <100.00%> (+0.01%)	⬆️

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[perrygreenfield]

LGTM, but way too many lines of code to review ;-) (i.e., anything more than 0)

[mcara]

LGTM, but way too many lines of code to review ;-) (i.e., anything more than 0)

Sorry!