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.

aports/main/postmarketos-android-recovery-installer/APKBUILD

 pkgname=postmarketos-android-recovery-installer
-pkgver=0.1.7
+pkgver=0.1.8
 pkgrel=1
 pkgdesc="TWRP compatible postmarketOS installer script"
 url="https://postmarketos.org"
@@ -28,7 +28,7 @@ package() {
 }
 sha512sums="924f961e1a488134d265f43724d2b06a908ac1522706dc3f7118f0dec16453aa0a928fef6d9a31a6a747910df665e64f25c94c47a9e3f1d57db19abb0fd8210d  build_zip.sh
 6390fc1b1c7dc8e917bb675d2e59e479782ac7754e97c4781866360ff704afe2d04f173a0ac74e3129db45328fab4b6b77a8717ee2e17c6ff79febadaa8ea034  update-binary
-f19c15fd99cc806d088ddf9c954111e48635f971ca500beeaa4893eb25d19fe0601210a57e9ab1a1dc7a6d79a3154765e696ee3329bbb1875b6d6df36a7b3fb3  pmos_chroot
+2260c5c7bf069a9fa57af5fe343aa8a9da1e258faf9be1900abce0c5768abbec178ac51d3fc48648e9f2fe3bd8d1f4e71f6d4beea07b524aeb94fd054a6def46  pmos_chroot
 94d06a7a73f76eb737e65ca2828f3c59d9cc532f2d2ec200e1407cd8fc266be192e33a8baa923e460a4335c7f5fb5232675d2e2e98a7e1ed07a257c67a0a5c5d  pmos_install
-514881d7812197830869c73428acc88f8a94dd02184634406ffe47884cd21d7b02ecc7806dae39308f026b1dbfca9ceee714fdfdc2bebc3ebcdeed71865de2b2  pmos_install_functions
+4ba66b336372b23252673f0929ea4f706ec287b5a902c8cb392a9cafb832db445bb7aecc9a7fac34f855babba057c56e3b10c015a1d055a66f0bfd42fef5828e  pmos_install_functions
 c6355b6d823dac883e1a352f59a9a2199e2934d78a73df72dc3c4fc14ef93765a15179203d4a8a2ca0d841b63cd4c25c4689b63c8cf4d4da2bcec1f8ff76bff5  pmos_setpw"