UnicodeDecodeError is raised when a local traceback variable can't be decoded

[bhch]

Sometimes some local variables in a function may have certain byte data which can't be decoded to string (such as xsrf tokens).

Since we display all the local variables found in the traceback frames on the debugger page, all variables go through Tornado's xhtml_escape function. The variables which can't be decode will raise UnicodeDecodeError.