_sandboxchroot.py: Improve mount error reporting

buildstream/_sandboxchroot.py

@@ -46,8 +46,10 @@ class Mount():
         if options:
             argv.extend(['-o', options])
 
+        cmd = argv + [src, dest]
+
         process = subprocess.Popen(
-            argv + [src, dest],
+            cmd,
             stdout=subprocess.PIPE,
             stderr=subprocess.PIPE
         )
@@ -56,17 +58,18 @@ class Mount():
         status = process.poll()
 
         if status != 0:
-            raise ElementError('Mounting {} failed with exit code {}:\n{}'
-                               .format(src, status, err.decode('utf-8')))
+            raise ElementError('`{}` failed with exit code {}:\n{}'
+                               .format(' '.join(cmd), status, err.decode('utf-8')))
 
         return dest
 
     def _umount(self, path):
 
         args = self.platform.switch(linux='-l', default=[])
+        cmd = [utils.get_host_tool('umount'), args, path]
 
         process = subprocess.Popen(
-            [utils.get_host_tool('umount'), args, path],
+            cmd,
             stdout=subprocess.PIPE,
             stderr=subprocess.PIPE
         )
@@ -75,8 +78,8 @@ class Mount():
         status = process.poll()
 
         if status != 0:
-            raise ElementError('Unmounting {} failed with exit code {}:\n{}'
-                               .format(path, status, err.decode('utf-8')))
+            raise ElementError('`{}` failed with exit code {}:\n{}'
+                               .format(' '.join(cmd), status, err.decode('utf-8')))
 
     @contextmanager
     def mount(self, src, dest, mount_type=None, **kwargs):
@@ -102,6 +105,7 @@ class Mount():
 
         self._umount(dest)
 
+
 class SandboxChroot(Sandbox):
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)