Append detailed printing information when exec_sudo fails

This patch tries to add more detailed info by appending
error output to exec_sudo print.

In current implementation, only a simple static note `Exception:
exec_sudo failed` is printed:
```
INFO diskimage_builder.block_device.utils [-] Calling [sudo sgdisk
 /tmp/dib_image.jZaDPxtX/image0.raw -n 1:0:+550M -t 1:EF00 -c 1:ESP
 -n 2:0:+8M -t 2:EF02 -c 2:BSP -n 3:0:+2077M -t 3:8300 -c 3:root]
ERROR diskimage_builder.block_device.blockdevice [-] Create failed;
 rollback initiated
// ... ...
diskimage_builder.block_device.exception.BlockDeviceSetupException:
 exec_sudo failed
```

but the root reason is missing. We can’t get more error info to
make sure what the real root reason even a simple problem like
`command not found`, we have to reproduce locally and manually.

After this patch, the error message like:
```
INFO diskimage_builder.block_device.utils [-] Calling [sudo sgdisk
 /tmp/dib_image.jZaDPxtX/image0.raw -n 1:0:+550M -t 1:EF00 -c 1:ESP
 -n 2:0:+8M -t 2:EF02 -c 2:BSP -n 3:0:+2077M -t 3:8300 -c 3:root]
ERROR diskimage_builder.block_device.blockdevice [-] Create failed;
 rollback initiated
// ... ...
diskimage_builder.block_device.exception.BlockDeviceSetupException:
 exec_sudo failed: sudo: sgdisk: command not found
```
We can easily find the real problem and solve it.

Closes-Bug: #2024980

Change-Id: I9efcd9cb6621e6403df6de14f122b1cf371bd800

diskimage_builder/block_device/utils.py

@@ -136,7 +136,7 @@ def exec_sudo(cmd):
     proc.wait()
 
     if proc.returncode:
-        e = BlockDeviceSetupException("exec_sudo failed")
+        e = BlockDeviceSetupException("exec_sudo failed: %s" % out)
         e.returncode = proc.returncode
         e.cmd = ' '.join(sudo_cmd)
         e.output = out