osbuild: also print what export is availalble when one is not found

The current error message when an export is not found could be
improved by printing what exports are actually availalble to make
it easier for the user to e.g. spot typos.

test/run/test_main.py

@@ -0,0 +1,24 @@
+import json
+
+import pytest
+
+from .. import test
+
+jsondata = json.dumps({
+    "version": "2",
+    "pipelines": [
+        {
+            "name": "noop",
+        },
+        {
+            "name": "noop2",
+        },
+    ],
+})
+
+
+def test_exports_are_shown_on_missing_exports(capsys):
+    with pytest.raises(AssertionError):
+        with test.OSBuild() as osb:
+            osb.compile(jsondata, exports=["not-existing"])
+    assert "Export not-existing not found in ['noop', 'noop2']\n" in capsys.readouterr().out