[libcalamares] Be more verbose in error situations

- runCommand can return NoWorkingDirectory in multiple places,
   make sure the log contains a more specific reason.

src/libcalamares/utils/CalamaresUtilsSystem.cpp

@@ -114,14 +114,24 @@ System::mount( const QString& devicePath,
        const QString& options )
 {
     if ( devicePath.isEmpty() || mountPoint.isEmpty() )
+    {
+        if ( devicePath.isEmpty() )
+            cWarning() << "Can't mount an empty device.";
+        if ( mountPoint.isEmpty() )
+            cWarning() << "Can't mount on an empty mountpoint.";
+
         return static_cast<int>(ProcessResult::Code::NoWorkingDirectory);
+    }
 
     QDir mountPointDir( mountPoint );
     if ( !mountPointDir.exists() )
     {
         bool ok = mountPointDir.mkpath( mountPoint );
         if ( !ok )
+        {
+            cWarning() << "Could not create mountpoint" << mountPoint;
             return static_cast<int>(ProcessResult::Code::NoWorkingDirectory);
+        }
     }
 
     QString program( "mount" );
@@ -147,7 +157,10 @@ System::runCommand(
     QString output;
 
     if ( !Calamares::JobQueue::instance() )
+    {
+        cError() << "No JobQueue";
         return ProcessResult::Code::NoWorkingDirectory;
+    }
 
     Calamares::GlobalStorage* gs = Calamares::JobQueue::instance()->globalStorage();
     if ( ( location == System::RunLocation::RunInTarget ) &&