releasetools: Change the base_fs assertion into warnings.

commit f54216f2 packed the base_fs files
into target_files.zip and added assertion to ensure the existence of the
files. We don't want to fail the OTA generation for the target_files.zip
without the base_fs files. Change the assertion into warnings instead.

Bug: 28547368
Change-Id: I6fd758a0a4fdfff02d1640fa46cf43d971627e26