Skip incomplete database directories during discovery

[elianddb]

A half-created DB directory was treated as valid causing any INFORMATION_SCHEMA query to fail server-wide.

• Valid() now requires RSLoadErr == nil
• skip such dirs with a warning during scan
• surface RSLoadErr in the load-failure error
  Fix sql-server: one broken database dir in the data dir fails every INFORMATION_SCHEMA query server-wide #11206

[coffeegoddd]

@elianddb DOLT

read_tests	from_latency	to_latency	percent_change
covering_index_scan	2.3	2.35	2.17
groupby_scan	139.85	139.85	0.0
index_join	2.0	2.0	0.0
index_join_scan	1.34	1.34	0.0
index_scan	219.36	223.34	1.81
oltp_point_select	0.24	0.24	0.0
oltp_read_only	4.91	4.91	0.0
select_random_points	0.54	0.54	0.0
select_random_ranges	0.64	0.63	-1.56
table_scan	196.89	200.47	1.82
types_table_scan	450.77	442.73	-1.78

write_tests	from_latency	to_latency	percent_change
oltp_delete_insert	6.21	6.21	0.0
oltp_insert	3.13	3.13	0.0
oltp_read_write	11.24	11.24	0.0
oltp_update_index	3.3	3.3	0.0
oltp_update_non_index	2.97	2.97	0.0
oltp_write_only	6.32	6.21	-1.74
types_delete_insert	6.79	6.79	0.0

[coffeegoddd]

@elianddb DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
16d6f9e	ok	5937471

version	total_tests
16d6f9e	5937471

correctness_percentage
100.0

[coffeegoddd]

@elianddb DOLT

test_name	from_latency_p95	to_latency_p95	percent_change
tpcc-scale-factor-1	44.17	45.79	3.67

test_name	from_server_name	from_server_version	from_tps	to_server_name	to_server_version	to_tps	percent_change
tpcc-scale-factor-1	dolt	4f81841	52.95	dolt	80053f9	53.28	0.62

[coffeegoddd]

@elianddb DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
4ef8a2d	ok	5937471

version	total_tests
4ef8a2d	5937471

correctness_percentage
100.0

[coffeegoddd]

@elianddb DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
c2fa977	ok	5937471

version	total_tests
c2fa977	5937471

correctness_percentage
100.0

[coffeegoddd]

@elianddb DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
4ad8f11	ok	5937471

version	total_tests
4ad8f11	5937471

correctness_percentage
100.0

[coffeegoddd]

@elianddb DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
80053f9	ok	5937471

version	total_tests
80053f9	5937471

correctness_percentage
100.0