Removed error numbers in favour of literal error messages. The error numbers

Removed error numbers in favour of literal error messages. The error numbers
were harder to manage, made the code harder to read, and lead no less
verbose messages, since it was effort to add a new message. The new approach
has the drawback that it's more difficult to translate, but this won't
happen anyway.

git-svn-id: svn://svn.cc65.org/cc65/trunk@428 b7a2c559-68d2-44c3-8de9-860c34a00d81