Change {Source,Binary}ErrorHandler to C++ (#403)
Added some new classes:
* BinaryErrorHandler: abstract base class
* BinaryErrorHandlerFile: writes errors to a FILE*
* SourceErrorHandler: abstract base class
* SourceErrorHandlerNop: eats all errors
* SourceErrorHandlerFile: writes errors to a FILE*
There are some specific ones as well:
* BinaryErrorHandlerAssert: used in wasm-interp to handle writing out a
custom header with assertion "failures"; i.e. places where the error
handler should fire.
* BinaryErrorHandlerModule: used when parsing a .wast, but there's an
embedded binary file.
