Address review comments

- Make 'you can use x.py' less definitive, since we don't know if the
toolchain was installed from source
- format! -> to_string()

Note that this still re-orders imports because rustfmt really wants them
to change.

src/errors.rs

@@ -445,8 +445,8 @@ fn component_unavailable_msg(cs: &[Component], manifest: &Manifest, toolchain: &
 
 fn install_msg(bin: &str, toolchain: &str, is_default: bool) -> String {
     if Toolchain::is_custom_name(toolchain) {
-        return format!("\nnote: this is a custom toolchain, which cannot use `rustup component add`\n\
-        help: if you installed this toolchain from source with `rustup component link`, you can build the component with `x.py`");
+        return "\nnote: this is a custom toolchain, which cannot use `rustup component add`\n\
+        help: if you built this toolchain from source, and used `rustup component link`, then you may be able to build the component with `x.py`".to_string();
     }
     match component_for_bin(bin) {
         Some(c) => format!("\nTo install, run `rustup component add {}{}`", c, {