Create a SchemaParseError that the new parser should use.

[elle-j]

The new schema parser from #5178 is currently throwing either an Error or AssertionError for invalid schemas. A SchemaParseError that extends Error would provide a more suitable exception to throw.

Notes:

• Wherever assert() is used, the error object will have to be passed to it (rather than just the message).
• This change may require integrating with the new updates in Core error handling.