Change AttributeError to HTTPPreconditionFailed in FileManager

[masipcat]

The main problem with AttributeError is that the view crashes with 500 and the exception goes to sentry

[codecov-io]

Codecov Report

Merging #926 into master will increase coverage by 0.3%. The diff coverage is n/a.

@@           Coverage Diff            @@
##           master    #926     +/-   ##
========================================
+ Coverage    94.3%   94.5%   +0.3%     
========================================
  Files         333     333             
  Lines       29429   29429             
========================================
+ Hits        27730   27794     +64     
+ Misses       1699    1635     -64

Impacted Files	Coverage Δ
guillotina/files/manager.py	100% <ø> (ø)	:arrow_up:
guillotina/tests/fixtures.py	92.5% <0%> (+0.6%)	:arrow_up:
guillotina/db/factory.py	93% <0%> (+7.6%)	:arrow_up:
guillotina/db/storages/dummy.py	94.1% <0%> (+31.8%)	:arrow_up: