footprint editor: assign 3d model: file not found warning icon when file is definitely found

Description

Assigning an external 3d model to a footprint, using the "path alias" feature. Selecting a model from a 3d alias path works, and is very visible in the previewer, and can be aligned/rotated, but an icon shows up in the top list of model files, warning that the file cannot be found. (Presumably it's naiively assuming the content is a real path)

KiCad Version

Application: KiCad Footprint Editor

Version: 5.99.0-1.20210804git5bf0b99.fc33, release build

Libraries:
	wxWidgets 3.0.5
	libcurl/7.71.1 OpenSSL/1.1.1k-fips zlib/1.2.11 brotli/1.0.9 libidn2/2.3.2 libpsl/0.21.1 (+libidn2/2.3.0) libssh/0.9.5/openssl/zlib nghttp2/1.43.0

Platform: Linux 5.13.6-100.fc33.x86_64 x86_64, 64 bit, Little endian, wxGTK, gnome-xorg, x11

Build Info:
	Date: Aug  4 2021 17:13:59
	wxWidgets: 3.0.5 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
	Boost: 1.73.0
	OCC: 7.5.0
	Curl: 7.71.1
	ngspice: 33
	Compiler: GCC 10.3.1 with C++ ABI 1014

Build settings:
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON