richedit: Set error codes and stop parsing for some rtf syntax errors.

richedit: Set error codes and stop parsing for some rtf syntax errors.

Checks were added for hexadecimal values that did not have valid
characters, and for EOF received before the final closing brace of the
rich text stream.  The error values were tested on richedit versions 1,
2, 3 & 4.1, and they were all the same for these cases.