Add error message in case setuptools is not installed.

setup.py

@@ -34,10 +34,28 @@
 import os
 import sys
 import time
+import textwrap
 import warnings
 import subprocess
 
-from setuptools import setup
+def normalizeWhitespace(s):
+    return ' '.join(s.split())
+
+try:
+    from setuptools import setup
+except ImportError:
+    s = normalizeWhitespace("""Limnoria requires the setuptools package to
+    install. This package is pretty standard, and often installed alongside
+    Python, but it is missing on your system.
+    Try installing it with your package manager, it is usually called
+    'python3-setuptools'. If that does not work, try installing python3-pip
+    instead, either with your package manager or by following these
+    instructions: https://pip.pypa.io/en/stable/installation/ (replace
+    'python' with 'python3' in all the commands)""")
+    sys.stderr.write(os.linesep*2)
+    sys.stderr.write(textwrap.fill(s))
+    sys.stderr.write(os.linesep*2)
+    sys.exit(-1)
 
 warnings.filterwarnings('always', category=DeprecationWarning)
 
@@ -100,9 +118,6 @@ if sys.version_info < (3, 6, 0) \
 plugins = [s for s in os.listdir('plugins') if
            os.path.exists(os.path.join('plugins', s, 'plugin.py'))]
 
-def normalizeWhitespace(s):
-    return ' '.join(s.split())
-
 packages = ['supybot',
             'supybot.locales',
             'supybot.utils',