Fix silent failure on invalid redirect syntax

[mac-chaffee]

Hello! I'm a new (happy) Pico user who recently tried to set up a redirection for my pgs site like this: https://pico.sh/pgs#redirect-www-to-naked-domain

I found there was an off-by-one error in the parsing of _redirects files that caused the the destination path/URL must start with '/', 'http:' or 'https: validation to never trigger.

If there is no valid "To Part" in the string, but the "From Part" was valid, the previous code would use the "From Part" as the "To Part". For example, the version in the docs /* naked-domain.com 301 would be parsed as From: "/*" , To: "/*", Status: 301

The updated code throws an error, which I guess would become a 502 error. I've also updated the docs in another PR: picosh/docs#5

[neurosnap]

This looks great, thanks so much for your contribution!

[mac-chaffee]

There's a chance that some silently-failing lines are hiding in existing _redirects files that might start suddenly breaking sites. If you have access, maybe checking existing usages would be wise