More specific exception types and no more silent errors

[mathbou]

Hi there,

No new feature here, but a lot of comfort. Working using tk-core and the rest of the toolkit is very frustrating, as a lot of exceptions are catch silently and prints absolutely nothing to the log or the console. So this PR adds log to all silent exceptions.

Also, some except were missing an Exception type, and others were using the bare Exception instead of a more precise one, it's taken care of. Due to the code complexity, a lot of exceptions still need/use the Exception type, but it would be a good thing to get rid of them.

As a "bonus", the QtImporter now raises an ImportError instead of returning a bunch of None.

[codecov]

Codecov Report

Attention: Patch coverage is 37.98450% with 80 lines in your changes missing coverage. Please review.

Project coverage is 73.43%. Comparing base (43814ac) to head (fe031a9).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
python/tank/platform/engine.py	53.33%	7 Missing ⚠️
python/tank/bootstrap/async_bootstrap.py	0.00%	6 Missing ⚠️
python/tank/util/metrics.py	68.42%	6 Missing ⚠️
.../tank/authentication/interactive_authentication.py	20.00%	4 Missing ⚠️
python/tank/commands/misc.py	0.00%	4 Missing ⚠️
python/tank/descriptor/io_descriptor/git.py	0.00%	4 Missing ⚠️
python/tank/util/qt_importer.py	69.23%	4 Missing ⚠️
python/tank/commands/move_pc.py	0.00%	3 Missing ⚠️
python/tank/commands/push_pc.py	0.00%	3 Missing ⚠️
python/tank/descriptor/io_descriptor/git_branch.py	25.00%	3 Missing ⚠️
... and 22 more

❗ There is a different number of reports uploaded between BASE (43814ac) and HEAD (fe031a9). Click for more details.

HEAD has 7 uploads less than BASE

Flag	BASE (43814ac)	HEAD (fe031a9)
	10	3

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #868      +/-   ##
==========================================
- Coverage   79.87%   73.43%   -6.44%     
==========================================
  Files         198      198              
  Lines       20719    20756      +37     
==========================================
- Hits        16550    15243    -1307     
- Misses       4169     5513    +1344     

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