Fix: --overwrite backs up old install dir, but it gets destroyed anyways (#25583)

* Make sure PackageInstaller does not remove the just-restored
  install dir after failure in spack install --overwrite
* Remove cryptic error message and rethrow actual error

lib/spack/llnl/util/filesystem.py

@@ -692,7 +692,7 @@ def replace_directory_transaction(directory_name, tmp_root=None):
 
     try:
         yield tmp_dir
-    except (Exception, KeyboardInterrupt, SystemExit) as e:
+    except (Exception, KeyboardInterrupt, SystemExit):
         # Delete what was there, before copying back the original content
         if os.path.exists(directory_name):
             shutil.rmtree(directory_name)
@@ -701,10 +701,7 @@ def replace_directory_transaction(directory_name, tmp_root=None):
             dst=os.path.dirname(directory_name)
         )
         tty.debug('DIRECTORY RECOVERED [{0}]'.format(directory_name))
-
-        msg = 'the transactional move of "{0}" failed.'
-        msg += '\n    ' + str(e)
-        raise RuntimeError(msg.format(directory_name))
+        raise
     else:
         # Otherwise delete the temporary directory
         shutil.rmtree(tmp_dir)