add an error message if a technical asset is missing which is used inside a communication link list, instead of crash

[CMon]

During the creation of a large thread model I might have forgotten to define something, which led to:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0xd8 pc=0x6f3278]

goroutine 1 [running]:
github.com/threagile/threagile/pkg/model.ParseModel(0xc0000c5188, 0xc0000bb400, 0xc000282270, 0xc000221920)
        /app/pkg/model/parse.go:397 +0x54d8
github.com/threagile/threagile/pkg/model.ReadAndAnalyzeModel(0xc0000c5188, {0xf45368, 0xc0005a8456})
        /app/pkg/model/read.go:41 +0x3a5
github.com/threagile/threagile/internal/threagile.(*Threagile).initAnalyze.func1(0xc0004b2d00?, {0xda0c41?, 0x4?, 0xda0c45?})
        /app/internal/threagile/analyze.go:21 +0xec
github.com/spf13/cobra.(*Command).execute(0xc0000ce608, {0xc000533340, 0x4, 0x4})
        /go/pkg/mod/github.com/spf13/cobra@v1.8.0/command.go:983 +0xaca
github.com/spf13/cobra.(*Command).ExecuteC(0xc0000ce308)
        /go/pkg/mod/github.com/spf13/cobra@v1.8.0/command.go:1115 +0x3ff
github.com/spf13/cobra.(*Command).Execute(...)
        /go/pkg/mod/github.com/spf13/cobra@v1.8.0/command.go:1039
github.com/threagile/threagile/internal/threagile.(*Threagile).Execute(0xc00009cf00)
        /app/internal/threagile/threagile.go:16 +0x25
main.main()
        /app/cmd/threagile/main.go:12 +0x32

After the fix it reports:

failed to read and analyze model: unable to parse model yaml: missing target technical asset "missing-asset-id" for communication link: "Missing Asset Comm Name"

[ezavgorodniy]

lgtm