feat, test, refactor: more useful assertion failure msgs

[rwalle61]

feat:

• assertion failure messages now much more informative and more consistent with each other (see comparison below)
• changed some negated assertions to pass instead of fail.
  E.g. previously expect(res).to.not.satisfyApiSpec would fail if res matched no responses in your API spec. expect(res).to.not.satisfyApiSpec meant "expect res to match but not satisfy a response defined in your API spec".
  Now expect(res).to.not.satisfyApiSpec means "expect res to neither match nor satisfy a response defined in your API spec". So if res matches no responses in your API spec, then expect(res).to.not.satisfyApiSpec passes.

test:

• added a few test cases
• updated the test openapi specs correspondingly

refactor:

• moved assertions into assertions dir
• throw ValidationErrors instead of generic Errors
• rather than pass error.message through to the user, construct a more informative message at the chai assertion layer.

Comparison: assertion failure messages now much more informative and more consistent with each other:
expect(response).to.satisfyApiSpec:

expect(object).to.satisfySchemaInApiSpec:

[codecov]

Codecov Report

Merging #67 into master will not change coverage by %.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #67   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           12        15    +3     
  Lines          232       281   +49     
=========================================
+ Hits           232       281   +49     

Impacted Files	Coverage Δ
lib/classes/AbstractResponse.js	100.00% <ø> (ø)
lib/assertions/satisfyApiSpec.js	100.00% <100.00%> (ø)
lib/assertions/satisfySchemaInApiSpec.js	100.00% <100.00%> (ø)
lib/classes/AbstractOpenApiSpec.js	100.00% <100.00%> (ø)
lib/classes/OpenApi2Spec.js	100.00% <100.00%> (ø)
lib/classes/OpenApi3Spec.js	100.00% <100.00%> (ø)
lib/classes/errors/ValidationError.js	100.00% <100.00%> (ø)
lib/index.js	100.00% <100.00%> (ø)
... and 1 more

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