pgsql: Make a cleanup pass over error reports in tsearch code.

Log Message:
-----------
Make a cleanup pass over error reports in tsearch code. Use ereport
for user-facing errors, fix some poor choices of errcode, adhere to
message style guide.

Modified Files:
--------------
pgsql/src/backend/commands:
tsearchcmds.c (r1.7 -> r1.8)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tsearchcmds.c?r1=1.7&r2=1.8)
pgsql/src/backend/tsearch:
dict_thesaurus.c (r1.9 -> r1.10)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tsearch/dict_thesaurus.c?r1=1.9&r2=1.10)
spell.c (r1.7 -> r1.8)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tsearch/spell.c?r1=1.7&r2=1.8)
ts_parse.c (r1.5 -> r1.6)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tsearch/ts_parse.c?r1=1.5&r2=1.6)
pgsql/src/backend/utils/adt:
tsquery.c (r1.11 -> r1.12)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/tsquery.c?r1=1.11&r2=1.12)
tsquery_cleanup.c (r1.7 -> r1.8)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/tsquery_cleanup.c?r1=1.7&r2=1.8)
tsvector.c (r1.9 -> r1.10)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/tsvector.c?r1=1.9&r2=1.10)
tsvector_op.c (r1.9 -> r1.10)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/tsvector_op.c?r1=1.9&r2=1.10)
tsvector_parser.c (r1.4 -> r1.5)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/tsvector_parser.c?r1=1.4&r2=1.5)