sphinx: report errors when dependencies are not met

To build the Sphinx documentation, we have the following dependencies:
* sphinx
* sphinx_rtd_theme
* pyyaml

If any of these dependencies are missing, we might end up with some
cryptic error messages. This patch adds better error reporting when
dependencies are not met.

(From yocto-docs rev: 19df8d1ec56dc2ecb44122288cc53e84237fab69)

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/sphinx/yocto-vars.py

@@ -1,10 +1,19 @@
 #!/usr/bin/env python
 import re
-import yaml
+import sys
 
 import sphinx
 from sphinx.application import Sphinx
 
+# This extension uses pyyaml, report an explicit
+# error message if it's not installed
+try:
+    import yaml
+except ImportError:
+    sys.stderr.write("The Yocto Project Sphinx documentation requires PyYAML.\
+    \nPlease make sure to install pyyaml python package.\n")
+    sys.exit(1)
+
 __version__  = '1.0'
 
 # Variables substitutions. Uses {VAR} subst using variables defined in poky.yaml