Improve serverExecutablePath description and error when pointing to a directory

package.json

@@ -136,7 +136,7 @@
           "scope": "resource",
           "type": "string",
           "default": "",
-          "markdownDescription": "Manually set a language server executable. Can be something on the $PATH or a path to an executable itself. Works with `~,` `${HOME}` and `${workspaceFolder}`. **Deprecated scope**: This option will be set to `machine` scope in a future release, so it can be changed only globally, not per workspace."
+          "markdownDescription": "Manually set a language server executable. Can be something on the $PATH or the full path to the executable itself. Works with `~,` `${HOME}` and `${workspaceFolder}`. **Deprecated scope**: This option will be set to `machine` scope in a future release, so it can be changed only globally, not per workspace."
         },
         "haskell.serverExtraArgs": {
           "scope": "resource",

src/extension.ts

@@ -23,7 +23,7 @@ import { CommandNames } from './commands/constants';
 import { ImportIdentifier } from './commands/importIdentifier';
 import { DocsBrowser } from './docsBrowser';
 import { downloadHaskellLanguageServer } from './hlsBinaries';
-import { executableExists, ExtensionLogger, resolvePathPlaceHolders } from './utils';
+import { directoryExists, executableExists, ExtensionLogger, resolvePathPlaceHolders } from './utils';
 
 // The current map of documents & folders to language servers.
 // It may be null to indicate that we are in the process of launching a server,
@@ -103,10 +103,16 @@ function findManualExecutable(logger: Logger, uri: Uri, folder?: WorkspaceFolder
   }
   logger.info(`Trying to find the server executable in: ${exePath}`);
   exePath = resolvePathPlaceHolders(exePath, folder);
-  logger.info(`Location after path variables subsitution: ${exePath}`);
+  logger.info(`Location after path variables substitution: ${exePath}`);
 
   if (!executableExists(exePath)) {
-    throw new Error(`serverExecutablePath is set to ${exePath} but it doesn't exist and it is not on the PATH`);
+    let msg = `serverExecutablePath is set to ${exePath}`;
+    if (directoryExists(exePath)) {
+      msg += ' but it is a directory and the config option should point to the executable *full* path';
+    } else {
+      msg += " but it doesn't exist and it is not on the PATH";
+    }
+    throw new Error(msg);
   }
   return exePath;
 }

src/utils.ts

@@ -263,6 +263,10 @@ export function executableExists(exe: string): boolean {
   return out.status === 0 || (isWindows && fs.existsSync(exe));
 }
 
+export function directoryExists(path: string): boolean {
+  return fs.existsSync(path) && fs.lstatSync(path).isDirectory();
+}
+
 export function resolvePathPlaceHolders(path: string, folder?: WorkspaceFolder) {
   path = path.replace('${HOME}', os.homedir).replace('${home}', os.homedir).replace(/^~/, os.homedir);
   if (folder) {