bootstrap: Show target in "No such target exists" message

This makes it a little easier to pinpoint the issue.

Author: neuschaefer

src/bootstrap/src/core/sanity.rs

@@ -287,7 +287,7 @@ than building it.
 
             if !has_target {
                 panic!(
-                    "No such target exists in the target list,\n\
+                    "{target_str}: No such target exists in the target list,\n\
                      make sure to correctly specify the location \
                      of the JSON specification file \
                      for custom targets!\n\