Add better error message if parent path exists but isn't a directory

[JWCook]

Fixes #433

Instead of a confusing error message like:

AttributeError: 'SQLiteDict' object has no attribute '_local_context'`

Show this error message:

FileExistsError: Parent path exists and is not a directory: <path>. Please either delete the file or choose a different path.

This applies to SQLiteDict, SQLitePickleDict, and FileDict.

This also creates _local_context earlier in SQLiteDict.__init__() so any other potential initialization errors won't attempt to reference _local_context before it's created.

[codecov-commenter]

Codecov Report

Merging #434 (9f8027f) into master (e9f425d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #434   +/-   ##
=======================================
  Coverage   98.72%   98.72%           
=======================================
  Files          17       17           
  Lines        1173     1176    +3     
  Branches      144      144           
=======================================
+ Hits         1158     1161    +3     
  Misses          5        5           
  Partials       10       10           

Impacted Files	Coverage Δ
requests_cache/backends/sqlite.py	98.71% <100.00%> (+0.02%)	⬆️

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