Raise with string instead of object

[redcatbear]

Situation

Currently the implementation adds the error object as parameter when raising an error via the error() method.

While that sounds like a good idea at first glance in combination with overridden __tostring and __concat methods, this tactic fails where no explict or implicit type conversion happens.

So while an error is correctly printed for example, using a gmatch on a throw error will fail, complaining that the parameter was a table where a string was expected.

It is safer to explicity call tostring() and raising the result as an error.

Acceptance Criteria

1. raise() method raises error with generated error message as parameter instead of the error object.