Improve serverExecutablePath missing error message

Fixes #256

Author: lukel97

package.json

@@ -149,7 +149,7 @@
           "scope": "resource",
           "type": "string",
           "default": "",
-          "description": "Set the path to your language server executable, if it's not already on your $PATH. Works with ~, ${HOME} and ${workspaceFolder}."
+          "description": "Manually set a language server executable. Can be something on the $PATH or a path to an executable itself. Works with ~, ${HOME} and ${workspaceFolder}."
         },
         "haskell.enable": {
           "scope": "resource",

src/extension.ts

@@ -80,7 +80,7 @@ function findManualExecutable(uri: Uri, folder?: WorkspaceFolder): string | null
   }
 
   if (!executableExists(exePath)) {
-    throw new Error('Manual executable missing');
+    throw new Error(`serverExecutablePath is set to ${exePath} but it doesn't exist and is not on the PATH`);
   }
   return exePath;
 }