Consistently use {!r}, not '{!r}' in diagnostics.

Consistently use {!r}, not '{!r}' in diagnostics.

This can cause confusion:
  * If the erroneous object is None, it is printed as 'None', which
    appears as a string (and could be the result of converting None
    to a string.)
  * If the erroneous object is a string, it is printed as ''<val>'',
    which is a rather strange combination of quotes.