Syntax error in input generates multiple error messages

[David Johnson-Davies]

When entering an expression via the Serial Monitor, a syntax error typically triggers multiple further errors, as the rest of the line is evaluated. For example, entering:

(defun add (x) (+ #\GH x))

produces the output:

300> (defun add (x) (+ #\GH 
Error: Unknown character
300> x)
Error: 'x' undefined
300> 
Error: Incomplete list
300> )
Error: Incomplete list
300> 

The first error should cause the rest of the input to be ignored.