DOC: make build fail when API doc build fails

Make sure we don't accidentally fail to build the API docs.

Add PYTHON environment variable to override Python executable in doc
Makefile, to make main Makefile.

For symptoms of problem, see: #527

Author: matthew-brett

doc/Makefile

@@ -1,6 +1,10 @@
 # Makefile for Sphinx documentation
 #
 
+# The Python executable to be used
+#
+PYTHON ?= python
+
 # You can set these variables from the command line.
 SPHINXOPTS    =
 SPHINXBUILD   = sphinx-build
@@ -98,4 +102,3 @@ gitwash-update:
 	python ../tools/gitwash_dumper.py source nibabel --github-user=nipy \
 		--project-url=http://nipy.org/nibabel \
 		--project-ml-url=https://mail.python.org/mailman/listinfo/neuroimaging
-

doc/tools/build_modref_templates.py

@@ -19,7 +19,7 @@
 
 def abort(error):
     print('*WARNING* API documentation not generated: %s' % error)
-    exit()
+    exit(1)
 
 
 if __name__ == '__main__':