Add list of error messages to README

[RangerMauve]

Hi,
I really like this library and appreciate all the work that has been put into it.
Currently I'm using it as part of an application that requires support for multiple languages. This means that using the message fields from errors directly isn't sufficient. What I need to do instead is look at what message is being generated and choose an appropriate string in the currently active language.
This means that I need to have a list of all possible error messages.
Crawling through the code to find them isn't difficult per se, however I think it would be more useful for people in similar situations to just look it up in the docs.
For a start, I've just added a list of all the messages in calls to the error() function, in the future it may be useful to add comments for what causes the error message (though I find it pretty obvious already)

Hopefully this is something that will be useful to the project. If not, at least I have the list for my own reference now. 😄

[LinusU]

This is awesome 🙌 thank you!