Don't overwrite data fields from inner errors

[ronenh]

When adding an inner error using .Err(err) we "merge" the data fields from the inner error to the outer and in the process may overwrite the fields of the outer error, including the msg field.
There is no reason to do that. The inner error's data fields remain in the inner error and get formatted correctly when calling .Error() on the outer error.

[coveralls]

Pull Request Test Coverage Report for Build 13444376735

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 65.886%

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Totals
Change from base Build 13418861578:	0.2%
Covered Lines:	197
Relevant Lines:	299

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💛 - Coveralls