make: document how error handling should be done correctly

Right now, when a variable expression cannot be parsed, the result of
calling Var_Subst is a string containing garbage, and no error is
reported.  In addition, there are some silent errors that are not
reported at all.  This combination makes it difficult to change the
error handling without introducing subtle breakage in some edge cases.

An example for garbage output is in varmod-subst-regex.mk, in target
mod-regex-compile-error.

No functional change.

(rillig)