FIX: Return error codes in dicomfs, rather than raise

[DimitriPapadopoulos]

Illegal raise

Illegal class 'int' raised; will result in a TypeError being raised instead.

I believe the intent was to return, not to raise. @chrispycheng Can you confirm?

[codecov]

Codecov Report

Merging #1048 (497b189) into master (75af371) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #1048   +/-   ##
=======================================
  Coverage   92.26%   92.26%           
=======================================
  Files         100      100           
  Lines       12205    12205           
  Branches     2136     2136           
=======================================
  Hits        11261    11261           
  Misses        616      616           
  Partials      328      328           

Impacted Files	Coverage Δ
nibabel/cmdline/dicomfs.py	18.96% <0.00%> (ø)

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[chrispycheng]

@DimitriPapadopoulos you got it! nice catch, thanks.

[effigies]

Thanks! Test failures are unrelated.