kbuild: improve error from file2alias

The original errormessage was just plain unreadable.

Sample error message after this update (not for real - I provoked it):

FATAL: drivers/net/s2io: sizeof(struct pci_device_id)=33 is not a modulo of the
size of section __mod_pci_device_table=160.
Fix definition of struct pci_device_id in mod_devicetable.h

Before a warning was generated - this is now a fatal error.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>