live: improve exception handling in image_create

Reduce the scope of the try except block that controls the case
of deleting the image backup in case of error. Now it only covers
the section of code after backup creation and up to image
verification. Check when the Exception is an OgError to raise
with added context.

Prevent the deletion of the target image in case of error before
the backup creation.

Bundle the backup creation on its own try except block to give
more feedback on a failed backup creation.

Enables a better error management allowing unhandled
exceptions to be reported properly.

src/live/ogOperations.py

@@ -421,7 +421,6 @@ class OgLiveOperations:
         if ogRest.terminated:
             return
 
-        try:
         diskname = get_disks()[disk-1]
         cxt = fdisk.Context(f'/dev/{diskname}', details=True)
         pa = None
@@ -457,9 +456,13 @@ class OgLiveOperations:
 
         logfile = open('/tmp/command.log', 'wb', 0)
 
+        try:
             if os.path.exists(image_path) and backup:
                 shutil.move(image_path, f'{image_path}.ant')
+        except OSError as e:
+            raise OgError(f'Cannot create backup for {image_path}: {e}') from e
 
+        try:
             p1 = Popen(cmd1, stdout=PIPE, stderr=logfile)
             p2 = Popen(cmd2, stdin=p1.stdout)
             p1.stdout.close()
@@ -493,7 +496,11 @@ class OgLiveOperations:
                 shutil.move(f'{image_path}.ant', image_path)
 
             self._restartBrowser(self._url)
+
+            if isinstance(e, OgError):
                 raise OgError(f'Failed to create image for {fstype} filesystem in device {padev}: {e}') from e
+            else:
+                raise
 
         try:
             st = os.stat(image_path)