model: fixup "infeasible" messages

model: fixup "infeasible" messages

These messages should be much more compact and targeted. Currently, we
either get no infeasibility messages (when verbose=0) or we get messages
every time is_infeasible() is executed. Neither is helpful for debugging,
since the infeasibility may be temporary, as we choose a new path for
execution on the fly. So instead, just print the infeasibility message
at the end of infeasible executions.

Also, rather than spread the messages across multiple lines, just put
them together into a single string.