fix panic for unexpected json file format

[jennifersp]

returns error message instead of panic

[fulghum]

Fix looks great! I just had one note to update the dolt table import docs, too, so that the JSON format we require is documented there, too, in addition to in this new error message.

[fulghum]

Looks great! Thanks for iterating on the details.

[github-actions]

@coffeegoddd DOLT

test_name	detail	row_cnt	sorted	mysql_time	sql_mult	cli_mult
batching	LOAD DATA	10000	1	0.05	1
batching	batch sql	10000	1	0.08	1.75
batching	by line sql	10000	1	0.07	2
blob	1 blob	200000	1	1.01	3.32	3.61
blob	2 blobs	200000	1	0.99	4.39	4.82
blob	no blob	200000	1	1	1.67	1.78
col type	datetime	200000	1	0.97	1.92	2.14
col type	varchar	200000	1	0.76	2.24	2.7
config width	2 cols	200000	1	0.84	1.6	1.75
config width	32 cols	200000	1	1.89	1.85	2.64
config width	8 cols	200000	1	1.02	2.2	2.26
pk type	float	200000	1	0.95	1.42	1.61
pk type	int	200000	1	0.84	1.6	1.75
pk type	varchar	200000	1	1.67	1.95	2.05
row count	1.6mm	1600000	1	6.46	1.76	1.95
row count	400k	400000	1	1.63	1.71	1.9
row count	800k	800000	1	3.27	1.72	1.91
secondary index	four index	200000	1	4.48	1.42	1.46
secondary index	no secondary	200000	1	0.99	1.62	1.91
secondary index	one index	200000	1	1.29	1.72	1.95
secondary index	two index	200000	1	2.38	1.47	1.66
sorting	shuffled 1mm	1000000	0	6.37	1.98	1.97
sorting	sorted 1mm	1000000	1	5.82	2.13	2.3

[github-actions]

@coffeegoddd DOLT

name	mean	mean_mult
dolt_blame_basic	43.1	1.33
dolt_blame_commit_filter	242.44	3.03
dolt_commit_ancestors_commit_filter	0.47	1.02
dolt_commits_commit_filter	0.47	1.02
dolt_diff_log_join_from_commit	137.07	1.44
dolt_diffs_commit_filter	0.54	1.06
dolt_diff_log_join_to_commit	135.51	1.42
dolt_history_commit_filter	0.77	1.38
dolt_diff_table_from_commit_filter	0.88	1.24
dolt_diff_table_to_commit_filter	0.88	1.29
dolt_log_commit_filter	0.49	1.02