Improve error message when opening an unexisting file

(for Camille)

Right now, if you try to open a trace file that doesn't exist (eg vite foo), the information window says:

File opened: foo

0 errors and 1 warnings were found during parsing.

Errors and warnings can be found in log.txt

There is no container. The trace can not be drawn.

The log file contains:

File foo


Warnings :
0 : open file error

Your trace has 0 errors and 1 warnings.

And on the console, there is:

Cannot open file foo

This could be more explicit with a an error window saying the file does not exist.