Return errors to Lua rather than dieing outright

[alerque]

See for example this comment: sile-typesetter/sile#872 (comment)

The drawimage() function and several others have the potential to run the ERROR() function which spits out a cryptic message and then dies hard. This is not friendly at all to applications which can't catch and handle such a problem — not even to error themselves with a friendlier message.

The library should be throwing an error that can be caught with pcall() in Lua, not just aborting. This probably means the exit(1) needs to be replaced in error.c with something more friendly to the lua module interface.