Expose validation error when creating extension instance

[pedroscaff]

When we create an extension instance and there is a validation issue coming from the extension, we currently cannot inform the user about it since EM doesn't return e.g. a bad request error for it.
Example error message returned:

failed to add instance for extension "row-level-security-lua": error executing statement 'CREATE VIRTUAL SCHEMA "v_schema_name" USING "EXA_EXTENSIONS"."RLS_ADAPTER" WITH SCHEMA_NAME = 'Schema Name' LOG_LEVEL='DEBUG'': E-EGOD-11: execution failed with SQL error code '42500' and message 'schema "v_schema_name" already exists (Session: 1797399225720176641)'