OpenEuphoria

Yeah, that's going to be a problem - Edita won't get an error code from eui.exe and hence will not attempt to open any ex.err file that gets created. What you need is an app along the lines of (warning completely untested):
1) Either set up an eu.cfg file (probably easier) or code something like:

object path 
constant eu4 = "E:\euphoria-40b4\" 
    path = getenv("PATH") 
    if atom(path) then path = "" end if 
    if match(lower(eu4&"\\bin"),lower(path))=0 then 
        path &= ";"&eu4&"\\bin" 
        setenv("PATH",path) 
    end if 
    setenv("EUDIR",eu4) 
    setenv("EUINC",...) 

2) code to run and return the exit code (which you'll want to compile):

function flatten(sequence s) 
sequence res, si 
    res = "" 
    for i=1 to length(txtlines) do 
        si = s[i] 
        if find(' ',si) and not find('\"',si) then 
            si = '\"'&si&'\"' 
        end if 
        res &= si&' ' 
    end for 
    return res 
end function 
 
sequence cl = command_line() 
    cl[1] = eu4&"\\bin\\eui.exe" 
    abort(system_exec(flatten(cl),2) 

If I find time, I'll look at adding environment settings to Edita\Options\Extensions so you won't have to do any of this, though I can't promise that any time soon.

Regards,
Pete

PS. not sure why you said ".exwc" is really ".eui", can't you just add a new extension?

DOH - just after posting I realised that if you set up the eu.cfg file (in the same directory as eui.exe) you shouldn't need part 2, just invoke eui.exe directly.

PS. not sure why you said ".exwc" is really ".eui", can't you just add a new extension?