Improve CreateTopology error message when topology already exists

[strk]

At the moment calling CreateTopology with a name of an already existing topology makes a lot of noise. It would be nice to have a nicely readable error message instead.

The noisy part is the CONTEXT:

ERROR:  schema "t5859" already exists                                                          
CONTEXT:  SQL statement "                      
      CREATE SCHEMA t5859;   

as the context is a really big SQL query. The function could autonomously check or intercept the exception and be more succint instead.