feat(web/bubblegum): report some errors to the user via HTTP

We can actually catch some errors that may be generated in bubblegum
applications where we can report them to the user in a way that doesn't
require curl -vv:

* Type errors in the status argument: By removing yants completely we
  not only (presumably) gain some performance, but also the ability to
  return an internal server error on an unexpected type instead of
  throwing.

* User generated evaluation errors: by using builtins.tryEval we can
  catch throws and asserts the user inserted when generating the body
  and report to the user that something went wrong. To do: also support
  for the headers.

Change-Id: I8363b9825c6c730e624eb8016a5482d63cbc1890
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2849
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

web/bubblegum/examples/hello.nix

@@ -33,6 +33,18 @@ let
         No coffee, I'm afraid
       '';
     };
+    "/type-error" = {
+      status = 666;
+      title = "bad usage";
+      content = ''
+        Never gonna see this.
+      '';
+    };
+    "/eval-error" = {
+      status = "OK";
+      title = "evaluation error";
+      content = builtins.throw "lol";
+    };
   };
 
   notFound = {