Detect if .installed.cfg could not be loaded

We need special care because configparser ignores nonexisting arguments
by default.

nxdbom/__init__.py

@@ -89,8 +89,10 @@ def bom_software(installed_software_path): # -> {} (name,kind) -> PkgInfo
         '__buildout_space_v__': '\v',
     }
     idb = configparser.ConfigParser(defaults=_)
-    idb.read('%s/.installed.cfg' % installed_software_path)
-
+    installed_cfg = '%s/.installed.cfg' % installed_software_path
+    _ = idb.read(installed_cfg)
+    if not _:
+        raise RuntimeError("Cannot load %r" % installed_cfg)
 
     for s in idb.sections():
         if s == 'buildout':

nxdbom/nxdbom_test.py

@@ -22,7 +22,7 @@ import nxdbom
 import pytest
 
 import os
-from os.path import dirname
+from os.path import dirname, exists
 
 
 @pytest.mark.parametrize('url,nameok,verok', [
@@ -339,6 +339,14 @@ def test_bom_software(tmpdir, build, bomok):
     assert nxdbom.fmt_bom(bom) == bomok
 
 
+# loading non-existing .installed.cfg -> error
+def test_bom_software_eexist():
+    ne = '/nonexisting'
+    assert not exists(ne)
+    with pytest.raises(RuntimeError, match="Cannot load '%s/.installed.cfg'" % ne):
+        nxdbom.bom_software(ne)
+
+
 # ---- txtar ----
 
 # txtar_* provide support for archives in txtar format