recovery zip: support twrp.fstab filename in addition to recovery.fstab (fix for compatibility with TWRP on sony-yuga)

Fixes #1636 (closed)

Fixes a broken error message (instead of checking for code 255 just check for any error code).

TWRP 3.2.0 uses "twrp.fstab" instead of "recovery.fstab", this fix makes pmbootstrap support both.

Tested on samsung-kminilte with TWRP 3.2.3(twrp.fstab) and 3.1.1(recovery.fstab). I verified the files are actually named that way in /etc for both versions.