HTC10 (pme) partition `/boot` not mounted

When tetherback tries to backup the partitions, it tries to unmount them before backing them up. `/boot` partition is not mounted which crashes tetherback when it's trying to unmount the partition.
Checking if a partition is really mounted before trying to unmount it solved the issue for me

Log of error:
```
(env) [chris@tp-w520 install]$ tetherback -N
tetherback v0.9.1
Found ADB version 1.0.40
Using default transfer method: adb exec-out pipe (--exec-out)
Device reports kernel 3.18.31-perf-gdad85f6-dirty
Device reports TWRP version 3.2.0-0
Reading partition map for mmcblk0 (66 partitions)...
partition map: 100%
Reading partition map for mmcblk0rpmb (0 partitions)...
partition map: 100%
Reading partition map for mmcblk1 (1 partitions)...
partition map: N/A% WARNING: partition mmcblk1p1 has no PARTNAME in its uevent file
Please report this issue at https://github.com/dlenski/tetherback/issues
Please post the entire output from tetherback!
partition map: 100%
Saving backup images in ./nandroid-backup-2018-11-28--12-56-04/ ...
Saving partition boot (mmcblk0p59), 64 MiB uncompressed...
Traceback (most recent call last):
File "/home/shared/dev/android/tetherback/install/env/bin/tetherback", line 11, in
sys.exit(main())
File "/home/shared/dev/android/tetherback/install/env/lib/python3.7/site-packages/tetherback/tetherback.py", line 359, in main
backup_partition(adb, partmap[standard], bp, args.transport, backupdir, args.verify)
File "/home/shared/dev/android/tetherback/install/env/lib/python3.7/site-packages/tetherback/tetherback.py", line 255, in backup_partition
if not really_umount(adb, '/dev/block/'+pi.devname, pi.mountpoint):
File "/home/shared/dev/android/tetherback/install/env/lib/python3.7/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 "/home/shared/dev/android/tetherback/install/env/lib/python3.7/site-packages/tetherback/adb_wrapper.py", line 35, in check_output
return sp.check_output(self.adbcmd(adbargs), universal_newlines=un, **kwargs)
File "/usr/lib64/python3.7/subprocess.py", line 389, in check_output
**kwargs).stdout
File "/usr/lib64/python3.7/subprocess.py", line 481, in run
output=stdout, stderr=stderr)
subprocess.CalledProcessError: Command '('adb', '-d', 'shell', 'umount /dev/block/mmcblk0p59 2>/dev/null && echo ok')' returned non-zero exit status 1.
```

output of `adb shell mount`:
```
~ # mount
rootfs on / type rootfs (rw,seclabel,size=1757884k,nr_inodes=439471)
tmpfs on /dev type tmpfs (rw,seclabel,nosuid,relatime,mode=755)
devpts on /dev/pts type devpts (rw,seclabel,relatime,mode=600)
proc on /proc type proc (rw,relatime,gid=3009,hidepid=2)
sysfs on /sys type sysfs (rw,seclabel,relatime)
selinuxfs on /sys/fs/selinux type selinuxfs (rw,relatime)
tmpfs on /tmp type tmpfs (rw,seclabel,relatime)
pstore on /sys/fs/pstore type pstore (rw,seclabel,relatime)
adb on /dev/usb-ffs/adb type functionfs (rw,relatime)
/dev/block/mmcblk0p61 on /cache type ext4 (rw,seclabel,relatime,data=ordered)
/dev/block/dm-0 on /data type ext4 (rw,seclabel,relatime,data=ordered)
/dev/block/dm-0 on /sdcard type ext4 (rw,seclabel,relatime,data=ordered)
/dev/block/mmcblk1p1 on /external_sd type vfat (rw,relatime,fmask=0000,dmask=0000,allow_utime=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro)
```