Give more useful comment for user to correct topology

when a node name used in link does not correspond to listed
node names, Bad link msg was thrown without info on which link
causes the problem.
This change gives additional info on link and catch the user error
more cleanly.

Signed-off-by: EstherLerouzic <esther.lerouzic@orange.com>

gnpy/core/convert.py

@@ -32,6 +32,7 @@
 from pathlib import Path
 from difflib import get_close_matches
 from gnpy.core.utils import silent_remove
+from gnpy.core.exceptions import NetworkTopologyError
 import time
 
 all_rows = lambda sh, start=0: (sh.row(x) for x in range(start, sh.nrows))
@@ -509,9 +510,12 @@ def parse_excel(input_filename):
     all_cities = Counter(n.city for n in nodes)
     if len(all_cities) != len(nodes):
         raise ValueError(f'Duplicate city: {all_cities}')
-    if any(ln.from_city not in all_cities or
-           ln.to_city   not in all_cities for ln in links):
-        raise ValueError(f'Bad link.')
+    bad_links = []
+    for lnk in links:
+        if lnk.from_city not in all_cities or lnk.to_city not in all_cities:
+            bad_links.append([lnk.from_city, lnk.to_city])
+    if bad_links:
+        raise NetworkTopologyError(f'Bad link(s): {bad_links}.')
 
     return nodes, links, eqpts