pgsql-server: Be more consistent about reporting SPI errors in the

Log Message:
-----------
Be more consistent about reporting SPI errors in the various PLs.
Create a shared function to convert a SPI error code into a string
(replacing near-duplicate code in several PLs), and use it anywhere
that a SPI function call error is reported.

Modified Files:
--------------
    pgsql-server/src/backend/executor:
        spi.c (r1.121 -> r1.122)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/executor/spi.c.diff?r1=1.121&r2=1.122)
    pgsql-server/src/include/executor:
        spi.h (r1.45 -> r1.46)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/executor/spi.h.diff?r1=1.45&r2=1.46)
    pgsql-server/src/pl/plperl:
        spi_internal.c (r1.2 -> r1.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/pl/plperl/spi_internal.c.diff?r1=1.2&r2=1.3)
    pgsql-server/src/pl/plpgsql/src:
        pl_exec.c (r1.109 -> r1.110)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/pl/plpgsql/src/pl_exec.c.diff?r1=1.109&r2=1.110)
        pl_handler.c (r1.21 -> r1.22)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/pl/plpgsql/src/pl_handler.c.diff?r1=1.21&r2=1.22)
    pgsql-server/src/pl/plpython:
        plpython.c (r1.50 -> r1.51)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/pl/plpython/plpython.c.diff?r1=1.50&r2=1.51)
    pgsql-server/src/pl/tcl:
        pltcl.c (r1.87 -> r1.88)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/pl/tcl/pltcl.c.diff?r1=1.87&r2=1.88)