Attempt to parse exceptions as DOMException type with errors.Is() support

[JohnStarich]

• Attempt to parse all js.Value.Call() errors as DOMExceptions
• Expose a NewDOMException() constructor and Is(error) bool method for easy errors.Is() comparisons
• Add more txn durability mode tests

[coveralls]

Pull Request Test Coverage Report for Build 4238814340

• 56 of 103 (54.37%) changed or added relevant lines in 10 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.9%) to 70.71%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
idb/db.go	3	4	75.0%
idb/open_db_request.go	1	2	50.0%
idb/transaction.go	8	9	88.89%
idb/cursor.go	4	6	66.67%
idb/db_factory.go	1	3	33.33%
idb/request.go	1	3	33.33%
idb/key_range.go	0	5	0.0%
idb/object_store.go	1	9	11.11%
idb/dom_exception.go	37	49	75.51%
idb/base_object_store.go	0	13	0.0%
<!--	Total:	56	103	54.37%	-->

Files with Coverage Reduction	New Missed Lines	%
idb/transaction.go	1	76.63%
idb/request.go	2	62.72%
<!--	Total:	3		-->

Totals
Change from base Build 4039037378:	0.9%
Covered Lines:	857
Relevant Lines:	1212

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💛 - Coveralls