Host runtime log messages

AMP runtime messages of the form:

```
message(`${foo} failed to ${bar}.`)
```

will be transpiled into:

```
linkedMessage('0', foo, bar)
```

'0' is indirected to a table linked elsewhere. The most appropriate place to put this is probably amp.dev.

amp.dev needs infra to serve these error messages and interpolate the message:

```
https://log.amp.dev/?e=0&s[]=amp-list&s[]=initialize&v=XXXX
```

Would then render HTML:

```
amp-list failed to initialize.
```

`v` is an identifier for runtime version since the message sets divert. The table will be static and JSON-formatted. The keys are error ids as base36 strings and the values are the messages in printf interpolation format.

```
{
'0': '%s failed to %s.',
// ...
}
```

Things we need to do:

- [ ] Download and cache versioned error message JSON from cdn.ampproject.org
- [ ] Implement log.amp.dev endpoint serving HTML error messages
- [ ] Design the error message page

//cc @alanorozco