Compiler error messages: reduce assertiveness of message E0384

This message is emitted as guidance by the compiler when a developer attempts to reassign a value to an immutable variable.  Following the message will always currently work, but it may not always be the best course of action; following the 'consider ...' messaging pattern provides a hint to the developer that it could be wise to explore other alternatives.

src/test/ui/liveness/liveness-assign/liveness-assign-imm-local-with-init.rs

@@ -1,6 +1,6 @@
 fn test() {
     let v: isize = 1; //~ NOTE first assignment
-                      //~| HELP make this binding mutable
+                      //~| HELP consider making this binding mutable
                       //~| SUGGESTION mut v
     v.clone();
     v = 2; //~ ERROR cannot assign twice to immutable variable `v`