[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
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html