Provide good error when going to invalid URL/path

optic/exceptions/BUILD

@@ -0,0 +1,8 @@
+package(
+    default_visibility = ["//:optic_internal"],
+)
+
+py_library(
+    name = "exceptions",
+    srcs = glob(["*.py"]),
+)

optic/exceptions/__init__.py

@@ -0,0 +1,15 @@
+class OpticException(Exception):
+    pass
+
+
+class OpticUserException(Exception):
+    def __str__(self):
+        return f"User Error: {super().__str__()}"
+
+
+class OpticInternalException(Exception):
+    pass
+
+
+class OpticDeveloperException(Exception):
+    pass

optic/lib/BUILD

@@ -6,6 +6,7 @@ py_library(
     name = "lib",
     srcs = glob(["*.py"]),
     deps = [
+        "//optic/exceptions",
         "//optic/state",
         "//protos:ui_py_proto",
     ],

optic/lib/runtime.py

@@ -2,8 +2,10 @@
 from typing import Any, Callable
 from .session import Session
 from optic.state.state import Store
+from optic.exceptions import OpticUserException
 
 Handler = Callable[[Any, Any], None]
+newline = "\n"
 
 
 class Runtime:
@@ -24,6 +26,16 @@ def reset_session(self):
         self._session = Session(Store(deepcopy(self._initial_state), self.get_handler))
 
     def run_path(self, path: str) -> None:
+        if path not in self._path_fns:
+            paths = list(self._path_fns.keys())
+            paths.sort()
+            raise OpticUserException(
+                f"""Accessed path: {path} not registered
+
+Try one of the following paths:
+{newline.join(paths)}
+                                     """
+            )
         self._path_fns[path]()
 
     def register_path_fn(self, path: str, fn: Callable[[], None]) -> None:

web/src/app/app.ts

@@ -19,6 +19,7 @@ import { ChannelService, ChannelStatus } from "../services/channel_service";
     font-size: 1.5rem;
     font-weight: 500;
     padding-bottom: 16px;
+    white-space: preserve;
   }
 
   .exception-title {