Add a new -k (--keep-going) option, like make has.

Previously, the default was to *always* keep going, which is actually not
usually what you want.  Now we actually exit correctly after an error.  Of
course you still might have multiple errors before existing if you were
building in parallel.

builder.py

@@ -116,7 +116,7 @@ def main(targets, buildfunc):
 
     def done(t, rv):
         if rv:
-            err('%s: exit code was %r\n' % (t, rv))
+            #err('%s: exit code was %r\n' % (t, rv))
             retcode[0] = 1
 
     for i in range(len(targets)):
@@ -127,6 +127,8 @@ def main(targets, buildfunc):
     
     for t in targets:
         jwack.get_token(t)
+        if retcode[0] and not vars.KEEP_GOING:
+            break
         lock = state.Lock(t)
         lock.trylock()
         if not lock.owned:
@@ -139,6 +141,8 @@ def main(targets, buildfunc):
     
     while locked or jwack.running():
         jwack.wait_all()
+        if retcode[0] and not vars.KEEP_GOING:
+            break
         if locked:
             t = locked.pop(0)
             lock = state.Lock(t)