exits with 255 when files not found

sbmodelr

@@ -138,15 +138,15 @@ def read_network(network_file):
             netf = f.read()
     except FileNotFoundError:
         print(f'ERROR: file {network_file} not found')
-        exit()
+        exit(-1)
     # create list for links
     ll = []
     # check if this is a digraph
     if( re.search(r'digraph\s*.*{', netf) ):
         ll.append((0,0))
     # check for the odd case of /* and */ appearing inside separate quotes and refuse to parse
     if( re.search(r'"[\s\S]*?\/\*[\s\S]*\*\/[\s\S]*?"', netf) ):
-        print(fr'{network_file} contains identifiers with \* and *\, and cannot be parsed')
+        print(fr' Warning: {network_file} contains identifiers with \* and *\, and cannot be parsed')
         return []
     # remove comments
     netf = re.sub(r'\s*//.*', '', netf)          # // comments
@@ -417,7 +417,7 @@ if( args.network ):
 seedmodel = load_model(seedmodelfile, remove_user_defined_functions=True)
 if( seedmodel is None):
     print(f'ERROR: {seedmodelfile} failed to load.\n')
-    exit()
+    exit(-1)
 
 # print some information about the model
 if( not args.quiet ):