Persist doc only when no error happens during parsing

[HiromuHota]

Description of the problems or issues

Is your pull request related to a problem? Please describe.

See #489.

Does your pull request fix any issue.

Fix #489.

Description of the proposed changes

Persist doc only when no error happens during parsing.

Test plan

Added a test that demonstrates #489.

Checklist

• [ ] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.
• [x] I have updated the CHANGELOG.rst accordingly.

[HiromuHota]

PyTorch 1.6.0 has been released just two days ago. Mypy complains that "unused 'type: ignore' comment" at torch.__version__. I couldn't track down which change at PyTorch v1.6.0 affects this, but removing that ignore comment satisfies Mypy.

[codecov-commenter]

Codecov Report

Merging #490 into master will increase coverage by 0.05%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #490      +/-   ##
==========================================
+ Coverage   85.84%   85.90%   +0.05%     
==========================================
  Files          88       88              
  Lines        4565     4568       +3     
  Branches      850      851       +1     
==========================================
+ Hits         3919     3924       +5     
+ Misses        464      463       -1     
+ Partials      182      181       -1     

Flag	Coverage Δ
#unittests	85.90% <100.00%> (+0.05%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/fonduer/features/featurizer.py	86.02% <100.00%> (ø)
src/fonduer/parser/parser.py	93.03% <100.00%> (+0.08%)	:arrow_up:
src/fonduer/utils/udf.py	88.57% <100.00%> (-0.11%)	:arrow_down:
src/fonduer/candidates/models/span_mention.py	84.11% <0.00%> (+1.86%)	:arrow_up:

[HiromuHota]

Please squash it when merging.