RDB$GET/SET_CONTEXT(): enclosing in apostrophes or double quotes of a missed namespace/variable will make output more readable

[pavel-zotov]

SQL> set heading off;
SQL> select rdb$get_context('SYSTEM', '') from rdb$database;

Statement failed, SQLSTATE = HY000
Context variable  is not found in namespace SYSTEM
SQL>
SQL> select rdb$get_context('', 'MY_VAR') from rdb$database;

Statement failed, SQLSTATE = HY000
Invalid namespace name  passed to RDB$GET_CONTEXT
SQL> show version;
ISQL Version: WI-T5.0.0.1001 Firebird 5.0 Beta 2
Server version:
Firebird/Windows/AMD/Intel/x64 (access method), version "WI-T5.0.0.1001 Firebird 5.0 Beta 2"

Output will be more readable if we enclose missed namespace/variable in apostrophes, i.e.:

Context variable '' is not found ...

or

Invalid namespace name '' passed ...

PS.
Discussed with Vlad after made test for #7537