Print an error if the godot-cpp submodule hasn't been initialized

This makes the error that is printed easier to understand, with
a command given as a solution to run.

The error message is also colored to match how core Godot displays
this kind of build errors.

SConstruct

@@ -1,5 +1,8 @@
 #!/usr/bin/env python
 import os
+import sys
+
+from methods import print_error
 
 
 def normalize_path(val, env):
@@ -48,6 +51,19 @@ compilation_db = env.CompilationDatabase(
 )
 env.Alias("compiledb", compilation_db)
 
+submodule_initialized = False
+dir_name = 'godot-cpp'
+if os.path.isdir(dir_name):
+    if os.listdir(dir_name):
+        submodule_initialized = True
+
+if not submodule_initialized:
+    print_error("""godot-cpp is not available within this folder, as Git submodules haven't been initialized.
+Run the following command to download godot-cpp:
+
+    git submodule update --init --recursive""")
+    sys.exit(1)
+
 env = SConscript("godot-cpp/SConstruct", {"env": env, "customs": customs})
 
 env.Append(CPPPATH=["src/"])