Warn users if they're not in a projectile project

[dantecatalfamo]

Currently if users aren't in a projectile project and try to run a platformio command, they receive a cryptic error about an argument not being stringp.

Instead now they will get a more helpful message telling them they're not in a projectile project.

[ZachMassia]

Looks good to me. Don't see any reason not to have better error messages.

I'm curious though, did you run in to a situation where platformio-mode was enabled when it shouldn't?

It's been a while now, but looking through everything it seems that platformio-conditionally-enable should prevent this.

[dantecatalfamo]

I found that projectile-verify-file will check for a file in the current directory even if it's not a "project". You can end up in a situation where platformio.ini exists in your current directory so platformio-mode gets enabled, but projectile-project-root still returns nil, resulting in an error whenever you try to run a command.

[dantecatalfamo]

It's not likely to happen since you don't edit c++ files in the root of a PlatformIO, but who knows ¯_(ツ)_/¯.

I found the error when I enabled platformio-mode manually in a PlatformIO project I thought was version controlled and I had no idea what was going wrong when I got the stringp error haha