GDScript: Improve error messages for invalid indexing

These errors are very common when using an invalid property name
or calling on an object of the wrong type, and the previous message
was a bit cryptic for users.

Co-authored-by: Rémi Verschelde <rverschelde@gmail.com>
Co-authored-by: golfinq <golfinqz@gmail.com>

modules/gdscript/tests/scripts/analyzer/errors/outer_class_constants.out

@@ -3,4 +3,4 @@ GDTEST_RUNTIME_ERROR
 >> on function: test()
 >> analyzer/errors/outer_class_constants.gd
 >> 8
->> Invalid get index 'OUTER_CONST' (on base: 'GDScript').
+>> Invalid access to property or key 'OUTER_CONST' on a base object of type 'GDScript'.

modules/gdscript/tests/scripts/analyzer/errors/outer_class_constants_as_variant.out

@@ -3,4 +3,4 @@ GDTEST_RUNTIME_ERROR
 >> on function: test()
 >> analyzer/errors/outer_class_constants_as_variant.gd
 >> 9
->> Invalid get index 'OUTER_CONST' (on base: 'GDScript').
+>> Invalid access to property or key 'OUTER_CONST' on a base object of type 'GDScript'.

modules/gdscript/tests/scripts/analyzer/errors/outer_class_instance_constants.out

@@ -3,4 +3,4 @@ GDTEST_RUNTIME_ERROR
 >> on function: test()
 >> analyzer/errors/outer_class_instance_constants.gd
 >> 8
->> Invalid get index 'OUTER_CONST' (on base: 'RefCounted (Inner)').
+>> Invalid access to property or key 'OUTER_CONST' on a base object of type 'RefCounted (Inner)'.

modules/gdscript/tests/scripts/analyzer/errors/outer_class_instance_constants_as_variant.out

@@ -3,4 +3,4 @@ GDTEST_RUNTIME_ERROR
 >> on function: test()
 >> analyzer/errors/outer_class_instance_constants_as_variant.gd
 >> 9
->> Invalid get index 'OUTER_CONST' (on base: 'RefCounted (Inner)').
+>> Invalid access to property or key 'OUTER_CONST' on a base object of type 'RefCounted (Inner)'.