tools: do not print error messages in verify_header() functions

default_image.c and socfpgaimage.c are the only image modules that print error
messages during header verification. The verify_header() is used to query if a
given image file is processed by the image format. Thus, if the image format
can't handle the file, it must simply return an error. Otherwise we pollute the
screen with errors messages until we find the image format that handle a given
image file.

Signed-off-by: Guilherme Maciel Ferreira <guilherme.maciel.ferreira@gmail.com>