Fix, ldap: safely retrieve error object attribute

[CameronNemo]

@AlexisLessard Please try to apply these changes and let me know if you still see the issue. I do not have access to an LDAP server to test with at the moment.

Closes #1118

[codecov]

Codecov Report

Merging #1119 into master will decrease coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1119      +/-   ##
==========================================
- Coverage   76.01%   75.99%   -0.03%     
==========================================
  Files          55       55              
  Lines        8598     8601       +3     
==========================================
  Hits         6536     6536              
- Misses       2062     2065       +3

Impacted Files	Coverage Δ
flask_appbuilder/security/manager.py	51.34% <0%> (-0.22%)	⬇️

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[AlexisLessard]

The following exception appears in my logs:

[2019-09-16 09:37:49,624] {manager.py:874} ERROR - {'desc': 'Bad search filter'}

The exception is written litteraly, and the app doesn't crash. User isn't impacted, and sysadmin can search for the error. Great job, @CameronNemo! Thx!

I've tested the change ad hoc, but if it follows in a following version, and when airflow updates the version is uses, I'll be able to use it in production.

[AlexisLessard]

If this PR is approved, you may close #1118

[dpgaspar]

Thank you (once more) @CameronNemo