Fix backup build erro exception class name

When backups fail to build, tempest is not displaying the correct
exception because the class name is not following the pattern we
expect it to follow.

This change fixes the class name for the share backups build error
exception.

Change-Id: I9553ba44544d84dbc452973252391967225c724e
diff --git a/manila_tempest_tests/share_exceptions.py b/manila_tempest_tests/share_exceptions.py
index 5b5ca18..7620109 100644
--- a/manila_tempest_tests/share_exceptions.py
+++ b/manila_tempest_tests/share_exceptions.py
@@ -88,5 +88,5 @@
                "status")
 
 
-class ShareBackupException(exceptions.TempestException):
+class ShareBackupBuildErrorException(exceptions.TempestException):
     message = ("Share backup %(backup_id)s failed and is in ERROR status")