Add error string label to bbr enabled metric

[stephen-soltesz]

We have learned that in practice, we see many status=false due to "bad file descriptor" errors. Unfortunately, this is not the error case we were originally interested in for the ndt7_measurer_bbr_enabled_total.

To be able to identify failures enabling BBR due to the module not being present, this change adds the error string to the metric labels. We are looking for the error "no such file or directory".

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This change is 

[stephen-soltesz]

FYI: @nkinkade

[coveralls]

Pull Request Test Coverage Report for Build 1609

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• 6 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.07%) to 81.658%

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Files with Coverage Reduction	New Missed Lines	%
ndt5/c2s/c2s.go	1	74.11%
ndt7/upload/sender/sender.go	5	65.91%
<!--	Total:	6		-->

Totals
Change from base Build 1607:	0.07%
Covered Lines:	1803
Relevant Lines:	2208

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💛 - Coveralls