Add warning on Aer and IBMQ ImportError (#2842)

* Add warning on Aer and IBMQ ImportError

This commit adds a warning whenever qiskit is imported if it can't
import qiskit-aer or qiskit-ibmq-provider. Right now if a user installs
terra and either does not install aer or the ibmq-provider, or also
equally as likely installs them but python packaging messes up the
namespace and there is no indication that these features are missing
until they go to use qiskit.Aer, or qiskit.IBMQ. To address this
commit adds a warning that will only be displayed once on the first
import of anything from the qiskit namespace in a python process, using
python's stdlib warning mechanism (which is configurable and
suppressible). This will hopefully remove the surprise when users try
to use Aer or IBMQ like in documentation and tutorials and they just
get an empty AttributeError.

Related #2483

* Update warning message

* Fix review comments and change warning class

qiskit/__init__.py

@@ -18,6 +18,7 @@
 """Main Qiskit public functionality."""
 
 import pkgutil
+import warnings
 
 # First, check for required Python and API version
 from . import util
@@ -50,12 +51,17 @@ from qiskit.providers.basicaer import BasicAer
 try:
     from qiskit.providers.aer import Aer
 except ImportError:
-    pass
+    warnings.warn('Could not import the Aer provider from the qiskit-aer '
+                  'package. Install qiskit-aer or check your installation.',
+                  ImportWarning)
 # Try to import the IBMQ provider if installed.
 try:
     from qiskit.providers.ibmq import IBMQ
 except ImportError:
-    pass
+    warnings.warn('Could not import the IBMQ provider from the '
+                  'qiskit-ibmq-provider package. Install qiskit-ibmq-provider '
+                  'or check your installation.',
+                  ImportWarning)
 
 from .version import __version__
 from .version import __qiskit_version__