core: update "missing dependencies" errors

Closes #363.

classes.py

@@ -20,7 +20,7 @@ from collections import defaultdict, deque
 try:
     import ircmatch
 except ImportError:
-    raise ImportError("Please install the ircmatch library and try again.")
+    raise ImportError("PyLink requires ircmatch to function; please install it and try again.")
 
 from . import world, utils, structures, __version__
 from .log import *

conf.py

@@ -8,7 +8,7 @@ It provides simple checks for validating and loading YAML-format configurations
 try:
     import yaml
 except ImportError:
-    raise ImportError("Please install PyYAML and try again.")
+    raise ImportError("PyLink requires PyYAML to function; please install it and try again.")
 
 import sys
 import os.path

coremods/login.py

@@ -4,7 +4,11 @@ login.py - Implement core login abstraction.
 
 from pylinkirc import conf, utils, world
 from pylinkirc.log import log
+
+try:
     from passlib.context import CryptContext
+except ImportError:
+    raise ImportError("PyLink requires passlib to function; please install it and try again.")
 
 pwd_context = CryptContext(["sha512_crypt", "sha256_crypt"],
                            all__vary_rounds=0.1,