eventstream: adds middleware to close input-writer in case of error

[skotambkar]

Changes

• Adds an unmarshal error middleware that will close the eventstream's inputwriter (io.writeCloser) in case server returns an error for the initial request handshake (before streaming payload).
• Updates code generation template - so this change affects lexruntimev2 and transcribestreamingservice.
• Added a handwritten test in transcribestreaming service pkg to test inputwriter is closed. (Uses mock http transport)
• Minor fix for validate response handler error to include previous errors
• Remove stale code

[jasdel]

Looks good, only minor comments