_includes: better error on missing include

Previously, a missing include would result in an error like this:

    Could not find file at not-a-file.include

Note that the file containing the include was not mentioned.

Now we get an error like this instead:

    element.bst [line 7 column 5]: Include block references a file that
    could not be found: 'not-a-file.include'.

1 files changed, 20 insertions, 0 deletions

diff --git a/tests/format/include.py b/tests/format/include.py
index 7344919ba..536f9faaf 100644
--- a/tests/format/include.py
+++ b/tests/format/include.py
@@ -1,4 +1,5 @@
 import os
+import textwrap
 import pytest
 from buildstream import _yaml
 from buildstream._exceptions import ErrorDomain, LoadErrorReason
@@ -27,6 +28,25 @@ def test_include_project_file(cli, datafiles):
     assert loaded['included'] == 'True'
 
 
+def test_include_missing_file(cli, tmpdir):
+    tmpdir.join('project.conf').write('{"name": "test"}')
+    element = tmpdir.join('include_missing_file.bst')
+
+    # Normally we would use dicts and _yaml.dump to write such things, but here
+    # we want to be sure of a stable line and column number.
+    element.write(textwrap.dedent("""
+        kind: manual
+
+        "(@)":
+          - nosuch.yaml
+    """).strip())
+
+    result = cli.run(project=str(tmpdir), args=['show', str(element.basename)])
+    result.assert_main_error(ErrorDomain.LOAD, LoadErrorReason.MISSING_FILE)
+    # Make sure the root cause provenance is in the output.
+    assert 'line 4 column 2' in result.stderr
+
+
 @pytest.mark.datafiles(DATA_DIR)
 def test_include_junction_file(cli, tmpdir, datafiles):
     project = os.path.join(str(datafiles), 'junction')