Add info to CannotPrecomp exception

The CannotPrecomp error message was LTA because it did not provide
actionable information to new users.  This revision clarifies that
users should remove the item that cannot be pre-compiled from a
Module.

src/core.c/Exception.pm6

@@ -343,7 +343,7 @@ my class X::Pragma::NoArgs is Exception {
 }
 my class X::Pragma::CannotPrecomp is Exception {
     has $.what = 'This compilation unit';
-    method message { "$.what may not be pre-compiled" }
+    method message { "$.what cannot be pre-compiled and thus cannot be used in a Module" }
 }
 my class X::Pragma::CannotWhat is Exception {
     has $.what;