panic when adding from a file that doesn't exists

[Gustavo Marin]

Describe the bug
If you try to add a profile from a file that doesn't exists, it panic.

Expected behavior

Check for the source file and fail more gracefully on missing file.

Additional context
Error:

hostctl add -p test --from .etchosts
Using hosts file: /etc/hosts
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x556854311397]

goroutine 1 [running]:
github.com/guumaster/hostctl/pkg/host.add(0x0, 0xc000133ca0, 0x0, 0x5568543c8ae0)
	github.com/guumaster/hostctl/pkg/host/add.go:75 +0xb7
github.com/guumaster/hostctl/pkg/host.AddFromFile(0xc000133d50, 0x55685431bb29, 0x7)
	github.com/guumaster/hostctl/pkg/host/add.go:39 +0xc3
github.com/guumaster/hostctl/cmd.glob..func1(0x5568544f7cc0, 0xc000068300, 0x0, 0x4, 0x0, 0x0)
	github.com/guumaster/hostctl/cmd/add.go:21 +0x17c
github.com/spf13/cobra.(*Command).execute(0x5568544f7cc0, 0xc0000682c0, 0x4, 0x4, 0x5568544f7cc0, 0xc0000682c0)
	github.com/spf13/cobra@v0.0.6/command.go:840 +0x455
github.com/spf13/cobra.(*Command).ExecuteC(0x5568544f99a0, 0x0, 0x0, 0x556800000000)
	github.com/spf13/cobra@v0.0.6/command.go:945 +0x319
github.com/spf13/cobra.(*Command).Execute(...)
	github.com/spf13/cobra@v0.0.6/command.go:885
github.com/guumaster/hostctl/cmd.Execute()
	github.com/guumaster/hostctl/cmd/root.go:35 +0x2f
main.main()
	github.com/guumaster/hostctl/main.go:10 +0x22