Rename E15: expected closing parenthesis to E116: …(same)
Do not error out for :'<NUL>
Add command-line parser

Known bug: `if|5|endif` is parsed to

    if
        5print
    endif

(put aside the fact that `if` without expression does not compile to syntox 
error).
Parse expressions in :if and some other commands

Also rename error to expr_error if appropriate: keep `error` names in this file 
pointing to CommandParserError’s only.