Remove error! macro

[lambda-fairy]

See https://github.com/lfairy/maud/blob/be88fc39d5d56370d3db020ef85f4f093ab45a60/maud_macros/src/parse.rs#L14-L19

This should really be a method on Parser instead.

Tasks:

• Add a Parser::error() method with a similar signature to the error!() macro
• Change all instances of error!(...) to return self.error(...)
• Remove the error!() macro

[alisha17]

I am working on this issue!