Fix #2197: Slightly cryptic error message for missing index.rst file

tests/test_build.py

@@ -12,6 +12,7 @@
 from six import BytesIO
 
 from textwrap import dedent
+from sphinx.errors import SphinxError
 
 from util import with_app, rootdir, tempdir, SkipTest, TestApp
 
@@ -73,6 +74,16 @@ def test_build_all():
         yield verify_build, buildername, srcdir
 
 
+@with_app(buildername='text')
+def test_master_doc_not_found(app, status, warning):
+    (app.srcdir / 'contents.txt').unlink()
+    try:
+        app.builder.build_all()
+        assert False  # SphinxError not raised
+    except Exception as exc:
+        assert isinstance(exc, SphinxError)
+
+
 @with_app(buildername='text', testroot='circular')
 def test_circular_toctree(app, status, warning):
     app.builder.build_all()