Catch exception if sqlmigrate fails

Because of missing constraint that Django cannot find and therefore drop.

django_migration_linter/migration_linter.py

@@ -199,6 +199,7 @@ class MigrationLinter(object):
             "Calling sqlmigrate command {} {}".format(app_label, migration_name)
         )
         dev_null = open(os.devnull, "w")
+        try:
             sql_statement = call_command(
                 "sqlmigrate",
                 app_label,
@@ -206,6 +207,16 @@ class MigrationLinter(object):
                 database=self.database,
                 stdout=dev_null,
             )
+        except ValueError:
+            logger.warning(
+                (
+                    "Error while executing sqlmigrate on (%s, %s). "
+                    "Continuing execution with empty SQL."
+                ),
+                app_label,
+                migration_name,
+            )
+            sql_statement = ""
         return sql_statement.splitlines()
 
     def _gather_migrations_git(self, git_commit_id):