settings.gradle: Better document and enforce min Gradle and JDK versions for build.
settings.gradle: Better document and enforce min Gradle and JDK versions for build.

* Create variables for min Gradle version for basic build, Java FX build
* Throw exception if Gradle less than minimum version
* Change existing println statements to print to stderr

natzei/bitcoinj,bitcoinj/bitcoinj,schildbach/bitcoinj,natzei/bitcoinj,kmels/bitcoinj,peterdettman/bitcoinj,kmels/bitcoinj,yenliangl/bitcoinj,peterdettman/bitcoinj,schildbach/bitcoinj,yenliangl/bitcoinj,bitcoinj/bitcoinj