--- a/launch	Mon May 20 07:34:46 2019 -0700
+++ b/launch	Mon May 20 08:40:56 2019 -0700
@@ -25,6 +25,6 @@
 app, errors = launch(fsroot=args.directory, urlroot=args.path)
 if errors:
     sys.stderr.write("{0}: {1} error{2} detected, aborting\n".format(
-        sys.argv[0], errors, "" if errors == 1 else "s"))
+        MYNAME, errors, "" if errors == 1 else "s"))
     sys.exit(1)
 app.run(host=args.bind, port=args.port)

--- a/tincan.py	Mon May 20 07:34:46 2019 -0700
+++ b/tincan.py	Mon May 20 08:40:56 2019 -0700
@@ -340,6 +340,11 @@
             return e
         except Exception as e:
             traceback.print_exc()
+            # Bottle doesn't allow error handlers to themselves cause
+            # errors, most likely as a measure to prevent looping. So
+            # this will cause a "Critical error while processing request"
+            # page to be displayed, and any installed error pages to be
+            # ignored.
             raise bottle.HTTPError(status=500, exception=e)
 
 class _TinCanRoute(object):
@@ -596,13 +601,23 @@
         """
         # Sanity checks
         if not self.urlroot.startswith("/"):
-            raise TinCanError("urlroot must be absolute")
+            self.errors = 1
+            self.logger("urlroot not absolute: {0!r}".format(self.urlroot))
+            return self
         if not os.path.isdir(self.fsroot):
-            raise TinCanError("no such directory: {0!r}".format(self.fsroot))
-        # Make WEB-INF, if needed
+            self.errors = 1
+            self.logger("no such directory: {0!r}".format(self.fsroot))
+            return self
+        # Make any needed directories. Refuse to launch things that don't
+        # contain WEB-INF, to prevent accidental launches of undesired
+        # directory trees containing sensitive files.
         winf = os.path.join(self.fsroot, _WINF)
+        if not os.path.isdir(winf):
+            self.errors = 1
+            self.logger("no WEB-INF directory in {0!r}".format(self.fsroot))
+            return self
         lib = os.path.join(winf, "lib")
-        for i in [ winf, lib ]:
+        for i in [ lib ]:
             if not os.path.isdir(i):
                 os.mkdir(i)
         # Add our private lib directory to sys.path