- Store std::string instead of std::unique_ptr<char[]> for the error
  message.
- Remove ErrorCode, which was just kSuccess and kError anyway. Error is
  now detected on whether error_msg_ is empty or not.
- Refactor constructors for perfect forwarding; this will allow us to
  implement Result<std::unique_ptr<X*>>.
- Refactor Decoder::toResult for perfect forwarding.
- Remove output operators (operator<<) for Result; it was only used in
  the error case anyway. Print error message directly instead.
  The operator was problematic since it assumed the existence of an
  output operator for every T which is used in Result<T>.
- Remove ModuleError and FunctionError, introduce general static
  Result<T>::Error method instead.

R=ahaas@chromium.org

Change-Id: I1e0f602a61ee9780fee2a3ed33147d431fb092ba
Reviewed-on: https://chromium-review.googlesource.com/472748
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44518}