better error message on download CI LLVM failure

Signed-off-by: onur-ozkan <work@onurozkan.dev>

src/bootstrap/src/core/download.rs

@@ -720,8 +720,10 @@ download-rustc = false
         if !tarball.exists() {
             let help_on_error = "ERROR: failed to download llvm from ci
 
-    HELP: old builds get deleted after a certain time
-    HELP: if trying to compile an old commit of rustc, disable `download-ci-llvm` in config.toml:
+    HELP: There could be two reasons behind this:
+        1) The host triple is not supported for `download-ci-llvm`.
+        2) Old builds get deleted after a certain time.
+    HELP: In either case, disable CI LLVM in your config.toml:
 
     [llvm]
     download-ci-llvm = false