[PostGIS] #5860: Improve CreateTopology error message when topology already exists

[PostGIS]

#5860: Improve CreateTopology error message when topology already exists
-------------------------+---------------------------
 Reporter:  strk         |      Owner:  strk
     Type:  enhancement  |     Status:  new
 Priority:  medium       |  Milestone:  PostGIS 3.5.3
Component:  topology     |    Version:  3.4.x
 Keywords:               |
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 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.
-- 
Ticket URL: <https://trac.osgeo.org/postgis/ticket/5860>
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