pgsql: Give a more user-friendly error message in situation where CREATE

Log Message:
-----------
Give a more user-friendly error message in situation where CREATE DATABASE
specifies a new default tablespace and the template database already has
some tables in that tablespace.  There isn't any way to solve this fully
without modifying the clone database's pg_class contents, so for now the
best we can do is issue a better error message.

Modified Files:
--------------
    pgsql/src/backend/commands:
        dbcommands.c (r1.144 -> r1.145)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/dbcommands.c.diff?r1=1.144&r2=1.145)
        tablespace.c (r1.11 -> r1.12)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablespace.c.diff?r1=1.11&r2=1.12)
    pgsql/src/include/commands:
        tablespace.h (r1.5 -> r1.6)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/tablespace.h.diff?r1=1.5&r2=1.6)