Raise typed errors

[brandur]

Changes all the places in the library where we raise generic errors to typed errors. These fall into two categories:

• When appropriate, I've tried to raise ArgumentError.
• Otherwise, I've defined a JsonSchema::Error from which all other thrown errors are inherited. The bang methods (expand!, parse!, and validate!) throw a new JsonSchema::AggregateError instead of just a RuntimeError with some strings concatenated. The new error is still a descendant of RuntimeError so this isn't a breaking change.

Fixes #49.

[skippy]

@brandur thank you kindly! quite helpful

[brandur]

@skippy Glad to hear!

Released as part of 0.12.0.