Improve feedback using @oracle:eval

If the given command fails (non-zerro exit code), show a configuration
error instead of a stack-trace.  The error include the command's stderr.

bugwarrior/config.py

@@ -56,20 +56,27 @@ def get_service_password(service, username, oracle=None, interactive=False):
         password = getpass.getpass(prompt)
     elif oracle.startswith('@oracle:eval:'):
         command = oracle[13:]
-        p = subprocess.Popen(
-            command,
-            shell=True,
-            stdout=subprocess.PIPE,
-            #stderr=subprocess.STDOUT
-        )
-        password = p.stdout.read()[:-1]
+        return oracle_eval(command)
 
     if password is None:
         die("MISSING PASSWORD: oracle='%s', interactive=%s for service=%s" %
             (oracle, interactive, service))
     return password
 
 
+def oracle_eval(command):
+    """ Retrieve password from the given command """
+    p = subprocess.Popen(
+        command, shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+    p.wait()
+    if p.returncode == 0:
+        return p.stdout.readline().strip()
+    else:
+        die(
+            "Error retrieving password: `{command}` returned '{error}'".format(
+                command=command, error=p.stderr.read().strip()))
+
+
 def load_example_rc():
     fname = os.path.join(
         os.path.dirname(__file__),