Error message geared toward serializer users

The previous message was targeted toward Serializer implementors, which is not
the group that will be seeing this message most often.

serde/src/ser/mod.rs

@@ -643,7 +643,7 @@ pub trait Serializer: Sized {
     {
         // TODO https://github.com/serde-rs/serde/issues/805
         // Remove this impl and force no_std formats to implement collect_str.
-        Err(Error::custom("Default impl of collect_str errors out for no_std builds"))
+        Err(Error::custom("this no_std format does not support serializing strings with collect_str"))
     }
 }