friendlier error messages when invoke/lessc are missing

and not needed.

IPython/html/tasks.py

@@ -1,10 +1,16 b''
 """invoke task file to build CSS"""
 
-from invoke import task, run
+from __future__ import print_function
+
 import os
+import sys
 from distutils.version import LooseVersion as V
 from subprocess import check_output
 
+from invoke import task, run
+from invoke.runner import Result
+from invoke.exceptions import Failure
+
 pjoin = os.path.join
 static_dir = 'static'
 components_dir = pjoin(static_dir, 'components')
@@ -13,6 +19,18 b' here = os.path.dirname(__file__)'
 min_less_version = '2.0'
 max_less_version = '3.0' # exclusive if string
 
+
+def _fail(msg=''):
+    """Fail a task, logging a message to stderr
+    
+    raises a special Failure Exception from invoke.
+    """
+    if msg:
+        print(msg, file=sys.stderr)
+    # raising a Failure allows us to avoid a traceback
+    # we only care about exited, but stdout, stderr, pty are required args
+    raise Failure(Result(stdout='', stderr='', pty=False, exited=1))
+
 def _need_css_update():
     """Does less need to run?"""
     
@@ -53,23 +71,30 b' def css(minify=False, verbose=False, force=False):'
         sourcemap = pjoin('style', "%s.min.css.map" % name)
         _compile_less(source, target, sourcemap, minify, verbose)
 
+
 def _compile_less(source, target, sourcemap, minify=True, verbose=False):
     """Compile a less file by source and target relative to static_dir"""
     min_flag = '-x' if minify else ''
     ver_flag = '--verbose' if verbose else ''
     
+    install = "(npm install -g 'less@<{}')".format(max_less_version)
     # pin less to version number from above
     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))
+        _fail("Unable to find lessc.  Rebuilding css requires less >= {0} and < {1} {2}".format(
+            min_less_version, max_less_version, install
+        ))
     out = out.decode('utf8', 'replace')
     less_version = out.split()[1]
     if min_less_version and V(less_version) < V(min_less_version):
-        raise ValueError("lessc too old: %s < %s. Use `$ npm install less@X.Y.Z` to install a specific version of less" % (less_version, min_less_version))
+        _fail("lessc too old: {} < {} {}".format(
+            less_version, min_less_version, install,
+        ))
     if max_less_version and V(less_version) >= V(max_less_version):
-        raise ValueError("lessc too new: %s >= %s. Use `$ npm install less@X.Y.Z` to install a specific version of less" % (less_version, max_less_version))
+        _fail("lessc too new: {} >= {} {}".format(
+            less_version, max_less_version, install,
+        ))
     
     static_path = pjoin(here, static_dir)
     cwd = os.getcwd()

setupbase.py

@@ -24,9 +24,10 b' from distutils.command.build_scripts import build_scripts'
 from distutils.command.install import install
 from distutils.command.install_scripts import install_scripts
 from distutils.cmd import Command
+from distutils.errors import DistutilsExecError
 from fnmatch import fnmatch
 from glob import glob
-from subprocess import check_call
+from subprocess import Popen, PIPE
 
 from setupext import install_data_ext
 
@@ -712,7 +713,15 b' class CompileCSS(Command):'
             cmd.append('--minify')
         if self.force:
             cmd.append('--force')
-        check_call(cmd, cwd=pjoin(repo_root, "IPython", "html"))
+        try:
+            p = Popen(cmd, cwd=pjoin(repo_root, "IPython", "html"), stderr=PIPE)
+        except OSError:
+            raise DistutilsExecError("invoke is required to rebuild css (pip install invoke)")
+        out, err = p.communicate()
+        if p.returncode:
+            if sys.version_info[0] >= 3:
+                err = err.decode('utf8', 'replace')
+            raise DistutilsExecError(err.strip())
 
 
 class JavascriptVersion(Command):
@@ -750,6 +759,7 b' def css_js_prerelease(command, strict=True):'
                 if strict:
                     raise
                 else:
-                    log.warn("Failed to build css sourcemaps: %s" % e)
+                    log.warn("rebuilding css and sourcemaps failed (not a problem)")
+                    log.warn(str(e))
             command.run(self)
     return DecoratedCommand