Improved error message when running bin/grav install instead of `bi…

…n/gpm install`, and also when running on a non-skeleton site #1027

```
$ bin/grav install
HINT Are you trying to install Grav? Grav is already installed. You
need to run this command only if you download a skeleton from GitHub
directly.

$ bin/grav install admin
HINT Are you trying to install a plugin or a theme? Make sure you use
bin/gpm install <something>, not bin/grav install. This command is only
used to install Grav skeletons.
```

CHANGELOG.md

@@ -8,6 +8,7 @@
 1. [](#improved)
     * Optionally remove unpublished pages from the translated languages, move into untranslated list [#1482](https://github.com/getgrav/grav/pull/1482)
     * Improved reliability of `hash` file-check method
+    * Improved error message when running `bin/grav install` instead of `bin/gpm install`, and also when running on a non-skeleton site [#1027](https://github.com/getgrav/grav/issues/1027)
 1. [](#bugfix)
     * Updated to latest Toolbox library to fix issue with some blueprints rendering in admin plugin [#1117](https://github.com/getgrav/grav-plugin-admin/issues/1117)
     * Fix output handling in RenderProcessor [#1483](https://github.com/getgrav/grav/pull/1483)

system/src/Grav/Console/Cli/InstallCommand.php

@@ -76,6 +76,13 @@ protected function serve()
             $this->config = Yaml::parse(file_get_contents($this->destination . $dependencies_file));
         } else {
             $this->output->writeln('<red>ERROR</red> Missing .dependencies file in <cyan>user/</cyan> folder');
+            if ($this->input->getArgument('destination')) {
+                $this->output->writeln('<yellow>HINT</yellow> <info>Are you trying to install a plugin or a theme? Make sure you use <cyan>bin/gpm install <something></cyan>, not <cyan>bin/grav install</cyan>. This command is only used to install Grav skeletons.');    
+            } else {
+                $this->output->writeln('<yellow>HINT</yellow> <info>Are you trying to install Grav? Grav is already installed. You need to run this command only if you download a skeleton from GitHub directly.');    
+            }
+
+            return;
         }
 
         // If yaml config, process