Don't print useless messages while parsing math macros

Don't print useless messages while parsing math macros

Math macros can be displayed on screen by providing a different
representation than the one used for latex output. This representation
is actually used by lyx even while it is being updated. This leads to
printing useless error messages on the terminal. For example, a macro
parameter has to be entered as \#1 and, if the macro is already used in
a math inset, lyx prints on terminal the error message "Math parse error:
missing token after \\" as soon as one hits the \ key, followed by
"MathMacroArgument::MathMacroArgument: wrong Argument id: -48" as soon as
one hits the # key. So, this is not a useful information and simply
clutters the terminal output. On the other hand, the input is sanitized
even if one stops input after hitting either \ or #, so that no further
messages are issued. Hence, those error messages are simply pointless.