fix(http): return error in case of panic

[fabiobozzo]

Address issue: https://github.com/ipfs/go-ipfs-cmds/issues/249

The new tests cover a case of sync http commands (e.g. /cat) and 'progress' commands (e.g. /add) panicking.

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[Stebalien]

Thanks!

[Stebalien]

(and thanks for writing a test)