File class always returns ERR_FILE_CANT_OPEN error instead of actual error

[andy-noisyduck]

Godot version:
3.1.1 (Mono)

OS/device including version:
Windows 10

Issue description:
When attempt to open a file using the File.open(...) method, if an error occurs (such as a missing file) the return error will always be ERR_FILE_CANT_OPEN. It is not possible to determine the more specific error. The expected errors ERR_FILE_NOT_FOUND for a missing file, or ERR_FILE_ALREADY_IN_USE for an exclusively locked file are never returned.

Steps to reproduce:
Open a "valid" but missing file path using File.open. Check the return error value. It will always be 12 (the error code for the generic ERR_FILE_CANT_OPEN error).

Minimal reproduction project:
Testing-FileOpen.zip

[mhilbrunner]

Yup. I'm currently working on a fix for this :)

[KoBeWi]

In 3.2 beta1, ERR_FILE_NOT_FOUND is returned correctly, but ERR_FILE_ALREADY_IN_USE is still not.

[timothyqiu]

ERR_FILE_ALREADY_IN_USE does not seem to be about "can't open an exclusively locked file". It's more like "there is already something in use", some examples:

godot/modules/mono/mono_gd/gd_mono_assembly.cpp

Lines 306 to 308 in 6e8eb0d

	Error GDMonoAssembly::load(bool p_refonly) {
	ERR_FAIL_COND_V(loaded, ERR_FILE_ALREADY_IN_USE);

godot/drivers/mpc/audio_stream_mpc.cpp

Lines 216 to 218 in 0b806ee

	Error AudioStreamMPC::_reload() {
	ERR_FAIL_COND_V(demux!=NULL, ERR_FILE_ALREADY_IN_USE);

[akien-mga]

Closing as "good enough" for now, but better error reporting for core APIs can be looked at for 4.0.