Failure in dialyzer check of value returned by the pre_dispatch/3 callback

Dialyzer is returning the following error when checking `lib/entry.ex`:
```
lib/entry.ex:291:pattern_match_cov
The pattern
variable_

can never match, because previous clauses completely cover the type
{:ok, _}.
```

This is happening because the [pre_dispatch/3](https://github.com/bitwalker/artificery/blob/master/lib/artificery.ex#L29) callback is defined like this:
```elixir
@callback pre_dispatch(Command.t, argv, options) :: {:ok, options} | no_return
```
But within the `Artificery.Entry` module the [function is called](https://github.com/bitwalker/artificery/blob/master/lib/entry.ex#L287-L297) assuming that some value other than `{:ok, options}` can be returned by it:
```elixir
case __MODULE__.pre_dispatch(command, argv, flags) do
{:ok, new_flags} ->
new_flags

other ->
Console.error(
"Expected {:ok, options} returned from #{__MODULE__}.pre_dispatch/3, got: #{
inspect(other)
}"
)
end
```

I'm not sure what the fix should be other than trying to find a way to disable the warning in dialyzer or crashing instead of calling `Console.error/1`.

Start by comparing the pre_dispatch/3 callback declaration in lib/artificery.ex around line 29 with its call in lib/entry.ex around lines 287-297. Run the Dialyzer check and inspect the reported pattern_match_cov warning. Done means the callback contract and caller agree and the warning is resolved without losing the intended error handling.

Written by the indexing model from the issue text.