Improve error messaging when encountering a primary key schema change during a merge.

[nicktobey]

When a merge encounters a table that can't be merged because its primary key changed, we now:

• Include the name of the affected table
• Indicate whether the primary key discrepancy is between the two branches or between one branch and the common ancestor.

[fulghum]

LGTM once you can get CI tests passing

[coffeegoddd]

@nicktobey DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
5dc2086	ok	5937457

version	total_tests
5dc2086	5937457

correctness_percentage
100.0

[coffeegoddd]

@nicktobey DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
bfe2e3f	ok	5937457

version	total_tests
bfe2e3f	5937457

correctness_percentage
100.0