Fix confusing error in abi_l1b reader when file fails to open

[djhoese]

Sometimes there are cases where the ABI L1B reader can't open a file and it never creates self.nc. In these cases the __del__ method was trying to access self.nc.close() and failing with an AttributeError. This AttributeError was getting through to the user and making the overall issue with their use of the reader confusing. This PR catches the AttributeError and stops it from being shown to the user. The rest of the reader should still properly report what went wrong with opening the file (not found, etc).

• Closes #xxxx
• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff
• Fully documented

[djhoese]

For reference the message was something like:

Traceback (most recent call last):
  File "/home/graemem/workspace/g2g_1_0_0/geo2grid_v_1_0_0/lib/python3.6/site-packages/satpy/readers/abi_l1b.py", line 220, in __del__
    self.nc.close()
AttributeError: 'NC_ABI_L1B' object has no attribute 'nc'

[codecov]

Codecov Report

Merging #615 into master will not change coverage.
The diff coverage is 50%.

@@           Coverage Diff           @@
##           master     #615   +/-   ##
=======================================
  Coverage   78.33%   78.33%           
=======================================
  Files         136      136           
  Lines       19714    19714           
=======================================
  Hits        15442    15442           
  Misses       4272     4272

Impacted Files	Coverage Δ
satpy/readers/abi_l1b.py	96.63% <50%> (ø)	⬆️

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[coveralls]

Coverage remained the same at 78.33% when pulling 2c16c2e on djhoese:bugfix-abi-l1b-bad-file into 57c7d21 on pytroll:master.