Add error reporting to generate_sample.sh on import failure.
Currently tools/conf/generate_sample.sh collect options from project
modules by importing each module and looking at it's global variables
for instances of classes derived from cfg.Opt (or lists of such
instances). Sometimes this import can fail due to various reasons,
cfg.DuplicateOptError for example. And if import fails no options
from imported module written to sample config file. Error reporting
will at least warn user that some of options didn't get into sample
config.