Remove error out for global parameters

[angelamayxie]

Erroring out is preventing gms from setting system variables when server starts (dolthub/go-mysql-server#3005) since AssignValues calls InitValue with global set to true.

[zachmu]

LGTM

[github-actions]

	Main	PR
covering_index_scan_postgres	346.48/s	337.34/s	-2.7%
index_join_postgres	150.85/s	151.23/s	+0.2%
index_join_scan_postgres	189.34/s	190.57/s	+0.6%
index_scan_postgres	12.47/s	12.29/s	-1.5%
oltp_point_select	2417.61/s	2467.01/s	+2.0%
oltp_read_only	1784.57/s	1788.54/s	+0.2%
select_random_points	114.46/s	116.30/s	+1.6%
select_random_ranges	133.24/s	126.36/s	-5.2%
table_scan_postgres	11.68/s	11.46/s	-1.9%
types_table_scan_postgres	5.44/s	5.40/s	-0.8%

[github-actions]

	Main	PR
Total	42090	42090
Successful	16646	16646
Failures	25444	25444
Partial Successes1	5539	5539

	Main	PR
Successful	39.5486%	39.5486%
Failures	60.4514%	60.4514%

Footnotes

1. These are tests that we're marking as Successful, however they do not match the expected output in some way. This is due to small differences, such as different wording on the error messages, or the column names being incorrect while the data itself is correct. ↩