ERC: recognize undefined netclasses

Description

With Kicad v7 (current nightly) it's possible to assign netclasses visually:

• with netclass-directive
• with additional netclass-field attached to a standard label

It's possible to write undefined netclass-strings into these netclass-directive / netclass-field. In this case the netclass of the wire resolves to the standard "default"-class, although it looks like a custom netclass is applied.

The ERC should catch these situation - normally it's either a mistake in writing or a forgotten netclass-definition.

Steps to reproduce

1. open attached project, open schematic: ERC_netclass.zip
2. look into Schematic Setup-->Project-->netclasses: there are 4 netclasses defined:
   • Default
   • netclass_A
   • netclass_B
   • netclass_C
3. close schematic-setup
4. look at the wire at R102: labeled "locallabel01", netclass-directive assigned with "netclass_01" --> this netclass is not defined! Select wire and look at statusbar --> netclass resolves to "Default"
5. look at the wire at R108: labeled "locallabel02", additional netclassfield attached with "useless_netclass" --> this netclass is not defined! Select wire and look at statusbar --> netclass also resolves to "Default"
6. run ERC - no complain regarding the netclasses

KiCad Version

Application: KiCad Schematic Editor (64-bit)

Version: (6.99.0-3433-g545212eb13), release build

Libraries:
	wxWidgets 3.1.7
	FreeType 2.11.1
	HarfBuzz 4.2.0
	FontConfig 2.14.0

Platform: Windows 10 (build 19044), 64-bit edition, 64 bit, Little endian, wxMSW

	wxWidgets: 3.1.7 (wchar_t,wx containers)
	Boost: 1.79.0
	OCC: 7.6.0
	Curl: 7.83.1-DEV
	ngspice: 37
	Compiler: Visual C++ 1929 without C++ ABI

Build settings:
	KICAD_SPICE=ON