Add `PatternError` exception.

[matulni]

Currently, most of the exceptions raised in :class: graphix.pattern.Pattern are of type ValueError. This generality hinders robust exception handling.

It may be worth introducing a new PatternError.
To be discussed:

• Exception class RunnabilityError introduced in Fix #193: add Pattern.check_runnability #364 should most likely inherit from PatternError.
• PatternError could have a limited number of possible reasons (implemented via Enum as in RunnabilityError or FlowError) or just be a generic handle as in OpenGraphError.