Better messages on nonexistent `-s ...` nodes

In heptc:

heptc -target c -hepts -s nonexistent test.ept
Warning: There is no node 'nonexistent' in module Test, the option -s was ignored.

In hepts:

$ hepts -mod Test -node nonexistent -exec test_c/sim
Error: There is no node 'nonexistent' in the module 'Test'.