new: dev: ``YAML`` merge error have better reporting

bin/compose

@@ -191,7 +191,13 @@ def merge_cli(*args):
     try:
         c = merge(*args)
     except NotImplementedError as e:
-        sys.stderr.write('%s. Conflicting key is %r. Values are:\n%s\n' % (e.args[0], e.args[1], e.args[2]))
+        sys.stderr.write('Merging Failed: %s.\n%s\n'
+            '  Values are:\n    %s\n'
+            % (e.args[0],
+               '  Conflicting key is %r.' % e.args[1] if e.args[1] else
+                   '  Conflict at base of structure.',
+               '\\n    '.join('v%d: %r' % (i, a)
+                         for i, a in enumerate(e.args[2]))))
         exit(1)
     if c is not None:
         print '%s' % yaml.dump(c, default_flow_style=False)