[xdl] Improve keytool errors

[EvanBacon]

When Java is not installed, the legacy keytool error messages are cryptic and hard to understand.
Resolves #3044

Running expo ba

before

If you don't know what this means, let us generate it! :) › Generate new keystore
Failed to generate Android Keystore, it will be generated on Expo servers during the build
keytool exited with non-zero code: 1

after

The operation couldn’t be completed. Unable to locate a Java Runtime.
Please visit http://www.java.com for information on installing Java.

Failed to generate Android Keystore, it will be generated on Expo servers during the build
keytool exited with non-zero code: 1