Fix returned error for Inject

[moorara]

I think when the passed SpanContext is not type of MockSpanContext, the error should be ErrInvalidSpanContext and not the ErrInvalidCarrier

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@659c906). Click here to learn what that means.
The diff coverage is 0%.

@@            Coverage Diff            @@
##             master     #216   +/-   ##
=========================================
  Coverage          ?   59.32%           
=========================================
  Files             ?       14           
  Lines             ?      831           
  Branches          ?        0           
=========================================
  Hits              ?      493           
  Misses            ?      298           
  Partials          ?       40

Impacted Files	Coverage Δ
mocktracer/mocktracer.go	95.83% <0%> (ø)

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