Add missing space in error message

A followup to 5414c42 which didn't fix every codepath.

Author: dralley

src/errors.rs

@@ -431,7 +431,7 @@ fn component_unavailable_msg(cs: &[Component], manifest: &Manifest, toolchain: &
             if toolchain.starts_with("nightly") {
                 "\nSometimes not all components are available in any given nightly. "
             } else {
-                ""
+                " "
             }
         );