OCaml virt-* tools: Handle pretty printing of exceptions through a common library function.
mllib: Wrap errors at column 76 instead of column 72.

Allows for slightly wider, better looking error messages.
mllib: error: Clearer message about how to enable debugging.
v2v: Update TODO.
v2v: Don't hard-code list of -i and -o options in help output.

We can now get it from Modules_list.