Reduce log level when block miner is interrupted

ethereum/blockcreation/src/main/java/tech/pegasys/pantheon/ethereum/blockcreation/AbstractBlockCreator.java

@@ -236,6 +236,11 @@ public void cancel() {
     isCancelled.set(true);
   }
 
+  @Override
+  public boolean isCancelled() {
+    return isCancelled.get();
+  }
+
   protected void throwIfStopped() throws CancellationException {
     if (isCancelled.get()) {
       throw new CancellationException();

ethereum/blockcreation/src/main/java/tech/pegasys/pantheon/ethereum/blockcreation/AsyncBlockCreator.java

@@ -15,4 +15,6 @@
 public interface AsyncBlockCreator extends BlockCreator {
 
   void cancel();
+
+  boolean isCancelled();
 }

ethereum/blockcreation/src/main/java/tech/pegasys/pantheon/ethereum/blockcreation/BlockMiner.java

@@ -67,14 +67,15 @@ public BlockMiner(
   public void run() {
 
     boolean blockMined = false;
-    while (!blockMined) {
+    while (!blockMined && !blockCreator.isCancelled()) {
       try {
         blockMined = mineBlock();
       } catch (final CancellationException ex) {
         LOG.debug("Block creation process cancelled.");
         break;
       } catch (final InterruptedException ex) {
-        LOG.error("Block mining was interrupted.", ex);
+        LOG.debug("Block mining was interrupted.", ex);
+        Thread.currentThread().interrupt();
       } catch (final Exception ex) {
         LOG.error("Block mining threw an unhandled exception.", ex);
       }