syz-fuzzer: communicate image testing errors to caller
syz-fuzzer: communicate image testing errors to caller

Prefix image testing errors with BUG: so that they are detected
by whoever is analyzing the output. Otherwise currently they are
dignosed as just "lost connection to test machine".

xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller