use specialized exception for interrupted sleep

src/main/java/org/cobbzilla/util/system/Sleep.java

@@ -25,7 +25,7 @@ public class Sleep {
         try {
             Thread.sleep(millis);
         } catch (InterruptedException e) {
-            throw new IllegalStateException("sleep interrupted (" + reason + ")");
+            throw new SleepInterruptedException("sleep interrupted (" + reason + ")");
         }
     }
 

src/main/java/org/cobbzilla/util/system/SleepInterruptedException.java

@@ -0,0 +1,7 @@
+package org.cobbzilla.util.system;
+
+public class SleepInterruptedException extends IllegalStateException {
+
+    public SleepInterruptedException(String msg) { super(msg); }
+
+}