Fix decodeUtf8

[mirokuratczyk]

The motivation is to make decoding errors easier to trace.

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For example, in the Haskell mongoDB library this call can result in a decoding error: Protocol.hs#L471.

The error would previously be:

Cannot decode byte '\\xc3': Data.Text.Internal.Encoding.decodeUtf8: Invalid UTF-8 stream

With this change it is a bit easier to reason about:

Data.Binary.Get.runGet at position 172: Data.Bson.Binary.getString: decodeUtf8 failed: Cannot decode byte '\\xc3': Data.Text.Internal.Encoding.decodeUtf8: Invalid UTF-8 stream\nCallStack (from HasCallStack):\n  error, called at libraries/binary/src/Data/Binary/Get.hs:351:5 in binary-0.8.5.1:Data.Binary.Get