update Loggers (#818)

* add warnings

* fix link

pytorch_lightning/loggers/neptune.py

@@ -12,7 +12,8 @@ from logging import getLogger
 try:
     import neptune
 except ImportError:
-    raise ImportError('Missing neptune package. Run `pip install neptune-client`')
+    raise ImportError('You want to use `neptune` logger which is not installed yet,'
+                      ' please install it e.g. `pip install neptune-client`.')
 
 from torch import is_tensor
 

pytorch_lightning/loggers/wandb.py

@@ -11,7 +11,8 @@ import os
 try:
     import wandb
 except ImportError:
-    raise ImportError('Missing wandb package.')
+    raise ImportError('You want to use `wandb` logger which is not installed yet,'
+                      ' please install it e.g. `pip install wandb`.')
 
 from .base import LightningLoggerBase, rank_zero_only