bug#6855: 24.0.50; Bug in tool bar label handling

[Eli Zaretskii]

> Date: Sun, 15 Aug 2010 12:21:37 +0200
> From: Jan Djärv <jan.h.d@swipnet.se>
> Cc: 6855@debbugs.gnu.org
> 
> If the documentation states that one should use STRING, and somebody puts in 
> nil or a lambda expression or a symbol, that is a usage error.  Being robust 
> against this kind of error by ignoring the faulty input just hides the error 
> and makes people think it is OK to misuse things.  Better then to crash, that 
> way action is usually taken at once.  Hidden errors can linger for years...

Crash or hide are not the only alternatives.  You can signal an error,
for instance.  Sometimes doing so is not a good idea, like in the
middle of redisplay (because displaying the error message reenters
redisplay again, and you have an infinite loop on your hands).  For
these situations, the solution is to display something prominent and
acutely visible instead of the invalid data, so that it stands out and
catches the user's eye.  For example, if the menu item is bad, display
something like "!!??GARBLED ITEM??!!" instead.

In general, I agree with Andreas: it is better not to crash, if we can
avoid that with a reasonable effort.