Fix crash when fdisk fails on usb disk device

In the event that `fdisk -l` fails on a partition or device (e.g., a
usb card reader has no media), don't crash the program.  Just return
None, since this is just a partition that the program can't use.

Fixes #180.

scripts/usb.py

@@ -219,7 +219,11 @@ def details_udev(usb_disk_part):
         gen.log("ERROR: Unknown disk/partition (%s)" % str(usb_disk_part))
         return None
 
-    fdisk_cmd_out = subprocess.check_output('fdisk -l ' + usb_disk_part, shell=True)
+    try:
+        fdisk_cmd_out = subprocess.check_output('fdisk -l ' + usb_disk_part, shell=True)
+    except subprocess.CalledProcessError:
+        gen.log("ERROR: fdisk failed on disk/partition (%s)" % str(usb_disk_part))
+        return None
 
     if b'Extended' in fdisk_cmd_out:
         mount_point = ''