Make runtime errors when someone forgets a direct failure persistence file

[sebas-zip]

This gives the user a runtime error when they want the test to use a Direct failure persistence file. Here is a code example:

 proptest! {
#![proptest_config(Config {
            // set failure persistance
            failure_persistence: Some(
                Box::new(FileFailurePersistence::Direct("past_failures_that_I_want_to_test.txt"))
            ),
            ..Default::default()
        })]
        
  #[test]
  fn  test_that_should_run_old_failures()....

Will give the correct behavior of:

---- <crate>::tests::proptest_test_that_should_run_old_failures stdout ----
thread <crate>::tests::proptest_test_that_should_run_old_failures' panicked at proptest-1.6.0/src/test_runner/failure_persistence/file.rs:399:17:
FileFailurePersistence::Direct set, but file does not exist. Please provide:
past_failures_that_I_want_to_test.txt
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Without this change if the user moves that file- this unit test will not raise an error. The intent by writing that statement is run this past failures before trying new random inputs. There should be a hard error to tell the user that something has gone wrong.

@matthew-russo @cameron1024. This is my first time contributing so please let me know anything I can do to make this PR easier to review.