better error message on download CI LLVM failure

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

Author: onur-ozkan

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: If trying to compile an old commit of rustc, rebase to recent commits. If you still get this error, your host triple most likely isn't supported for CI LLVM. Therefore, disable it in config.toml:
 
     [llvm]
     download-ci-llvm = false