XML read doesn't return error when file doesn't exist

[avilleret]

On Ubuntu, if path is a directory, I do receive a non-zero error code.
If it is a file it returns 0.
But if it point to nothing (i.e. the file doesn't exists) it also returns 0.
This is weird and thus I have no clue if the file correctly read.

I didn't test on other platform.
The code involved for this test is on JamomaPureData repo but something similar appear in JamomaMax code.
see : https://github.com/jamoma/JamomaPureData/blob/master/library/source/JamomaForPd.cpp#L130

AFAIK, this might be due to libxml xmlNewTextReaderFilename() method that doesn't return an error code when the file doesn't exist. So this is a bug of libxml2. But I think we can find a workaround.