[feature request] Handle `adb` errors gracefully.

[dreirund]

I have installed tetherback version 0.9.1.

Error messages from adb are not cleanly displayed but result in a python exception -- I think they should be handled gracefully within the python script, adb error cleanly displayed, and then the script to exit with an appropriate exit code, without throwing a python exception.

Two examples:

tetherback -0 -v (Android system was bootet, no recovery environment):

tetherback v0.9.1
Found ADB version 1.0.41
Using default transfer method: adb exec-out pipe (--exec-out)
Device reports kernel 3.18.140-Redux-V2.05/cea733f601
/system/bin/sh: twrp: inaccessible or not found
Traceback (most recent call last):
  File "/usr/bin/tetherback", line 33, in <module>
    sys.exit(load_entry_point('tetherback==0.9.1', 'console_scripts', 'tetherback')())
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/tetherback/tetherback.py", line 313, in main
    check_TWRP(p, adb, args.force)
  File "/usr/lib/python3.11/site-packages/tetherback/tetherback.py", line 88, in check_TWRP
    output = adb.check_output(('shell','twrp -v')).strip()
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/tetherback/adb_wrapper.py", line 35, in check_output
    return sp.check_output(self.adbcmd(adbargs), universal_newlines=un, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/subprocess.py", line 466, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/subprocess.py", line 571, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '('adb', '-d', 'shell', 'twrp -v')' returned non-zero exit status 127.

tetherback -v -M -D -R -C -X android_expand (recovery environment was bootet):

[...]
Backup plan:

PARTITION NAME    FILENAME                 FORMAT
----------------  -----------------------  -----------------
android_expand    android_expand.emmc.win  gzipped raw image
boot              boot.emmc.win            gzipped raw image
recovery          recovery.emmc.win        gzipped raw image
cache             cache.f2fs.win           tar -cz -p
system            system.ext4.win          tar -cz -p
userdata          data.ext4.win            tar -cz -p

Saving backup images in ./twrp-backup-2023-08-19--12-34-05/ ...
Saving partition android_expand (mmcblk1p2), 121924 MiB uncompressed...
Traceback (most recent call last):
  File "/usr/bin/tetherback", line 33, in <module>
    sys.exit(load_entry_point('tetherback==0.9.1', 'console_scripts', 'tetherback')())
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/tetherback/tetherback.py", line 338, in main
    backup_partition(adb, partmap[standard], bp, args.transport, backupdir, args.verify)
  File "/usr/lib/python3.11/site-packages/tetherback/tetherback.py", line 239, in backup_partition
    if not really_umount(adb, '/dev/block/'+pi.devname, pi.mountpoint):
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/tetherback/adb_stuff.py", line 32, in really_umount
    if adb.check_output(('shell','umount %s 2>/dev/null && echo ok' % dev)).strip():
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/tetherback/adb_wrapper.py", line 35, in check_output
    return sp.check_output(self.adbcmd(adbargs), universal_newlines=un, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/subprocess.py", line 466, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/subprocess.py", line 571, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '('adb', '-d', 'shell', 'umount /dev/block/mmcblk1p2 2>/dev/null && echo ok')' returned non-zero exit status 1.

Regards!