Annotate unpack* function errors with where the error happened.

[EricIO]

Here's a first stab at the code for #1576

This commit adds annotations to the error message when an error occurs during unpacking of a DNS message, the annotation will give detiail on where in the DNS message the unpacking error occured at. This helps to improve the debugability of such errors.

[miekg]

this looks good, however a whole bunch of

= RUN   TestTsigErrorResponse
    tsig_test.go:69: question.Qclass: %!w(<nil>)
--- FAIL: TestTsigErrorResponse (0.00s)
=== RUN   TestTsigBadTimeResponse
    tsig_test.go:98: question.Qclass: %!w(<nil>)
--- FAIL: TestTsigBadTimeResponse (0.00s)
=== RUN   TestTsigErrors
    tsig_test.go:140: expected an error 'dns: bad time' but got 'question.Qclass: %!w(<nil>)'
--- FAIL: TestTsigErrors (0.00s)

is popping up in the tests....

[EricIO]

this looks good, however a whole bunch of

= RUN   TestTsigErrorResponse
    tsig_test.go:69: question.Qclass: %!w(<nil>)
--- FAIL: TestTsigErrorResponse (0.00s)
=== RUN   TestTsigBadTimeResponse
    tsig_test.go:98: question.Qclass: %!w(<nil>)
--- FAIL: TestTsigBadTimeResponse (0.00s)
=== RUN   TestTsigErrors
    tsig_test.go:140: expected an error 'dns: bad time' but got 'question.Qclass: %!w(<nil>)'
--- FAIL: TestTsigErrors (0.00s)

is popping up in the tests....

Oops sorry forgot to add my latest changes...

[EricIO]

@miekg Does it look better now on your end as well?