Intermediary `SkipTestException`

[Mandera]

Create an intermediary inheritable class that we can use for EnvVar in ManderaGeneral/generalpackager#86

Clean up repr and str for it, made them match built-in exceptions:

from generalimport import MissingOptionalDependency

print(repr(AttributeError("asd")))
print(repr(MissingOptionalDependency("asd")))

print(repr(AttributeError()))
print(repr(MissingOptionalDependency()))

print(str(AttributeError("asd")))
print(str(MissingOptionalDependency("asd")))

print(str(AttributeError()))
print(str(MissingOptionalDependency()))

>>> AttributeError('asd')
>>> MissingOptionalDependency('asd')
>>> AttributeError()
>>> MissingOptionalDependency()
>>> asd
>>> asd
>>>   # Empty string
>>>   # Empty string