Improve error display in default error handler.

- Pretty-print the error message, and include the error type.  Previously, only
  the error arguments were printed so that e.g. `end-of-buffer' would be
  rendered just as "nil".  Pretty-printing makes the output of the default error
  handler consistent with default error output.

- Use `display-warning' rather than `message'.  Rationale: the error handler
  might be called many times in quick succession, overwriting previous messages.
  Also, the messages output by the error handler might be overwritten by
  messages emitted by other parts of the code (e.g. by websocket callbacks
  invoked after the error occurred.)

  Although past messages are always available in the *Messages* buffer, with
  `display-warning' errors are displayed more prominently since the *Warnings*
  buffer is unburied whenever new errors are printed.
Merge pull request #28 from jscheid/master

Improve error display in default error handler