Nicer error message when sphinx-build is not in PATH

__main__.py

@@ -42,7 +42,13 @@ def complete(args):
 def launch(args):
     import runpy
     sys.argv = args[1:]
-    runpy.run_path(args[1], run_name='__main__')
+    exe = args[1]
+    if exe.startswith(':'):
+        import shutil
+        exe = shutil.which(exe[1:])
+        if not exe:
+            raise SystemExit('{} not found in PATH'.format(args[1][1:]))
+    runpy.run_path(exe, run_name='__main__')
 
 
 def run_kitten(args):

docs/Makefile

@@ -3,7 +3,7 @@
 
 # You can set these variables from the command line.
 SPHINXOPTS    = -T $(FAIL_WARN)
-SPHINXBUILD   = python3 .. +launch $(shell which sphinx-build)
+SPHINXBUILD   = python3 .. +launch :sphinx-build
 SPHINXPROJ    = kitty
 SOURCEDIR     = .
 BUILDDIR      = _build