[dataimport] _create_copyfrom_buffer: raise ValueError if conversion cannot be performed
If the conversion fails, there is no reason to continue execution. One should
notify the error.
Continuing after a misconverted data could corrupt the database.
Related to #3845572
../../doc/book usr/share/doc/cubicweb-documentation
../../doc/html usr/share/doc/cubicweb-documentation
../../debian/cubicweb-doc usr/share/doc-base/cubicweb-doc