Warn developer when a template-tag package is bad

Fixes #497.

pipeline/templatetags/pipeline.py

 from __future__ import unicode_literals
 
+import logging
+
 from django.contrib.staticfiles.storage import staticfiles_storage
 
 from django import template
@@ -12,6 +14,7 @@ from ..conf import settings
 from ..packager import Packager, PackageNotFound
 from ..utils import guess_type
 
+logger = logging.getLogger(__name__)
 
 register = template.Library()
 
@@ -73,6 +76,7 @@ class StylesheetNode(PipelineMixin, template.Node):
         try:
             package = self.package_for(package_name, 'css')
         except PackageNotFound:
+            logger.warn("Package %r is unknown. Check PIPELINE_CSS in your settings.", package_name)
             return ''  # fail silently, do not return anything if an invalid group is specified
         return self.render_compressed(package, 'css')
 
@@ -101,6 +105,7 @@ class JavascriptNode(PipelineMixin, template.Node):
         try:
             package = self.package_for(package_name, 'js')
         except PackageNotFound:
+            logger.warn("Package %r is unknown. Check PIPELINE_JS in your settings.", package_name)
             return ''  # fail silently, do not return anything if an invalid group is specified
         return self.render_compressed(package, 'js')