Cause erlang:halt/[01] to actually halt

We do this by thawing a special java.lang.Error-derived exception 
erjng.ErjangHalt, which is handles specially in the top-level
frames of erlang processes.

Should fix issue 59 trifork#59
except we still need to make kilim not print the stack trace.

src/main/java/erjang/EProc.java

@@ -454,6 +454,9 @@ public void execute() throws Pausable {
 				result = e.reason();
 				death = e;
 
+			} catch (ErlangHalt e) {
+				throw e;
+
 			} catch (Throwable e) {
 
 				log.log(Level.SEVERE, "[java] exiting "+self_handle()+" with: ", e);
@@ -480,6 +483,8 @@ public void execute() throws Pausable {
 
 					try {
 						fun.invoke(this, new EObject[] { EString.fromString(msg) });
+					} catch (ErlangHalt e) {
+						throw e;
 					} catch (ThreadDeath e) {
 						throw e;
 					} catch (Throwable e) {
@@ -493,6 +498,9 @@ public void execute() throws Pausable {
 
 			do_proc_termination(result);
 
+		} catch (ErlangHalt e) {
+			throw e;
+
 		} catch (ThreadDeath e) {
 			throw e;
 

src/main/java/erjang/ErlangHalt.java

@@ -0,0 +1,6 @@
+package erjang;
+
+/** thrown when erlang:halt is invoked */
+public class ErlangHalt extends java.lang.Error {
+
+}

src/main/java/erjang/m/erlang/ErlProc.java

@@ -51,6 +51,7 @@
 import erjang.ErjangHibernateException;
 import erjang.ErlangException;
 import erjang.ErlangExit;
+import erjang.ErlangHalt;
 import erjang.ErlangUndefined;
 import erjang.FunID;
 import erjang.Import;
@@ -299,7 +300,7 @@ public static EObject halt(EProc proc, EObject value) {
 		}
 		ERT.shutdown();
 
-		return null;
+		throw new erjang.ErlangHalt();
 	}
 
 	@BIF