add check that dash was correctly imported and exit with helpful mess…

…age if failed (#39)

* add check that dash was imported and exit with useful message if not

* don't add print_function to namespace

* added test to check for sys.exit(1) when dash.py present

dash_html_components/__init__.py

@@ -1,8 +1,17 @@
+from __future__ import print_function as _
+
 import os as _os
 import sys as _sys
+
 import dash as _dash
+
 from .version import __version__
 
+if not hasattr(_dash, 'development'):
+    print("Dash was not successfully imported. Make sure you don't have a file "
+          "named \n'dash.py' in your current directory.", file=_sys.stderr)
+    _sys.exit(1)
+
 _current_path = _os.path.dirname(_os.path.abspath(__file__))
 
 _components = _dash.development.component_loader.load_components(