add instruction to pin lessc

IPython/html/fabfile.py

@@ -1,50 +1,50 b''
 """ fabfile to prepare the notebook """
 
 from fabric.api import local,lcd
 from fabric.utils import abort
 import os
 from distutils.version import LooseVersion as V
 from subprocess import check_output
 
 pjoin = os.path.join
 static_dir = 'static'
 components_dir = os.path.join(static_dir, 'components')
 
 min_less_version = '1.4.0'
 max_less_version = '1.5.0' # exclusive
 
 def css(minify=True, verbose=False):
     """generate the css from less files"""
     for name in ('style', 'ipython'):
         source = pjoin('style', "%s.less" % name)
         target = pjoin('style', "%s.min.css" % name)
         _compile_less(source, target, minify, verbose)
 
 def _to_bool(b):
     if not b in ['True', 'False', True, False]:
         abort('boolean expected, got: %s' % b)
     return (b in ['True', True])
 
 def _compile_less(source, target, minify=True, verbose=False):
     """Compile a less file by source and target relative to static_dir"""
     minify = _to_bool(minify)
     verbose = _to_bool(verbose)
     min_flag = '-x' if minify is True else ''
     ver_flag = '--verbose' if verbose is True else ''
     
     # pin less to 1.4
     try:
         out = check_output(['lessc', '--version'])
     except OSError as err:
         raise ValueError("Unable to find lessc.  Please install lessc >= %s and < %s " \
                          % (min_less_version, max_less_version))
     out = out.decode('utf8', 'replace')
     less_version = out.split()[1]
     if V(less_version) < V(min_less_version):
-        raise ValueError("lessc too old: %s < %s" % (less_version, min_less_version))
+        raise ValueError("lessc too old: %s < %s. Use `$ npm install lesscss@X.Y.Z` to install a specific version of less" % (less_version, min_less_version))
     if V(less_version) >= V(max_less_version):
-        raise ValueError("lessc too new: %s >= %s" % (less_version, max_less_version))
+        raise ValueError("lessc too new: %s >= %s. Use `$ npm install lesscss@X.Y.Z` to install a specific version of less" % (less_version, max_less_version))
     
     with lcd(static_dir):
         local('lessc {min_flag} {ver_flag} {source} {target}'.format(**locals()))