Add extra suggestions when Hex can't be installed, closes #10039

Author: josevalim

lib/mix/lib/mix/local.ex

@@ -161,14 +161,29 @@ defmodule Mix.Local do
         contents
 
       {:remote, message} ->
-        Mix.raise("""
-        #{message}
+        Mix.raise(
+          """
+          #{message}
 
-        Could not install #{name} because Mix could not download metadata at #{path}.
-        """)
+          Could not install #{name} because Mix could not download metadata at #{path}.
+          """ <> suggestions(name)
+        )
     end
   end
 
+  defp suggestions("Hex") do
+    """
+
+    Alternatively, you can compile and install Hex directly with this command:
+
+        mix archive.install github hexpm/hex branch latest
+    """
+  end
+
+  defp suggestions(_) do
+    ""
+  end
+
   defp parse_csv(body) do
     body
     |> :binary.split("\n", [:global, :trim])

lib/mix/lib/mix/tasks/local.hex.ex

@@ -11,6 +11,11 @@ defmodule Mix.Tasks.Local.Hex do
 
       mix local.hex
 
+  If installing a precompiled Hex does not work, you can compile and install
+  Hex directly with this command:
+
+      mix archive.install github hexpm/hex branch latest
+
   ## Command line options
 
     * `--force` - forces installation without a shell prompt; primarily