Handle weird xrefs during loading

[njbooher]

The importer initially ran into an issue trying to import xrefs on some features

https://db.scnbase.org/feature/?feature_id=215199

specifically these: GENE3D:G3DSA:1.10.287.770

This resolved it.

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the terms of the GNU General Public License version 3 as published by
the Free Software Foundation

I have read the CONTRIBUTING.rst file and understand that
TravisCI will be used to confirm the tests and flake8 style
checks pass with these changes.

I am happy be thanked by name in the CONTRIB.rst files,
and have added myself to the file as part of this pull request.
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[codecov-commenter]

Codecov Report

Merging #327 (97ad5a1) into master (72ba56c) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #327   +/-   ##
=======================================
  Coverage   68.71%   68.71%           
=======================================
  Files          30       30           
  Lines        4060     4060           
  Branches      235      235           
=======================================
  Hits         2790     2790           
  Misses       1211     1211           
  Partials       59       59           

Impacted Files	Coverage Δ
machado/loaders/feature.py	66.26% <100.00%> (ø)
machado/loaders/featureattributes.py	77.89% <100.00%> (ø)

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