Consistent error line reporting

[matteo-cristino]

The line where an error occur should always be reported in the same way, i.e. by adding to the trace

"[!] Error at Zencode line " .. linenum

Currently this is done only on runtime error, but this should be extend also to parsing error.
This is necessary to have better looking zencode error in slangroom and could also be usefull for other stuffs.