Add test for plugin install heuristic

core/src/main/java/org/elasticsearch/plugins/InstallPluginCommand.java

@@ -134,9 +134,6 @@ class InstallPluginCommand extends SettingCommand {
         }
     }
 
-    // protocols allowed for direct url installation
-    private static final List<String> URL_PROTOCOLS = Arrays.asList("http", "https", "file");
-
     private final OptionSpec<Void> batchOption;
     private final OptionSpec<String> arguments;
 

qa/evil-tests/src/test/java/org/elasticsearch/plugins/InstallPluginCommandTests.java

@@ -307,6 +307,12 @@ public class InstallPluginCommandTests extends ESTestCase {
         assertTrue(e.getMessage(), e.getMessage().contains("no protocol"));
     }
 
+    public void testUnknownPlugin() throws Exception {
+        Tuple<Path, Environment> env = createEnv(fs, temp);
+        UserError e = expectThrows(UserError.class, () -> installPlugin("foo", env.v1()));
+        assertTrue(e.getMessage(), e.getMessage().contains("Unknown plugin foo"));
+    }
+
     public void testPluginsDirMissing() throws Exception {
         Tuple<Path, Environment> env = createEnv(fs, temp);
         Files.delete(env.v2().pluginsFile());