Enhance Spectacles exceptions

[joshtemple]

This PR changes the attributes of SpectaclesException and its children.

Going forward, all SpectaclesException instances should have, at minimum, the following attributes:

• name: A lowercase, hyphenated, unique semantic ID for the error (e.g. unable-to-authenticate)
• title: A short, human-readable error message with a period and capitalized first letter
• detail: A human-readable message providing additional details and preferably offering suggestions for debugging or fixing the issue

Subclasses of SpectaclesException may define other attributes as needed. For example, LookerApiError accepts the requests response and parses out some useful data like the URL, method, and any data returned by the Looker API as JSON.

The remainder of the PR is refactoring existing error messages to match this new format.

[codecov]

Codecov Report

Merging #196 into master will increase coverage by 0.60%.
The diff coverage is 89.47%.

@@            Coverage Diff             @@
##           master     #196      +/-   ##
==========================================
+ Coverage   83.49%   84.09%   +0.60%     
==========================================
  Files          10       10              
  Lines         921      937      +16     
==========================================
+ Hits          769      788      +19     
+ Misses        152      149       -3     

Impacted Files	Coverage Δ
spectacles/cli.py	71.83% <ø> (ø)
spectacles/utils.py	85.36% <75.00%> (-0.69%)	⬇️
spectacles/client.py	91.52% <85.71%> (+1.58%)	⬆️
spectacles/validators.py	98.36% <87.50%> (+<0.01%)	⬆️
spectacles/exceptions.py	93.33% <100.00%> (+4.87%)	⬆️

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