docs/reference/fidl/language/error-catalog/_fi-0157.md

## fi-0157: Client/server end constraint must be a protocol {:#fi-0157}

The first constraint applied to either `client_end` or `server_end` must point
to a `protocol` definition:

{% include "docs/reference/fidl/language/error-catalog/label/_bad.md" %}

```fidl
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="tools/fidl/fidlc/tests/fidl/bad/fi-0157.test.fidl" exclude_regexp="\/\/ (Copyright 20|Use of|found in).*" %}
```

Change the constraint to point to a protocol instead:

{% include "docs/reference/fidl/language/error-catalog/label/_good.md" %}

```fidl
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="tools/fidl/fidlc/tests/fidl/good/fi-0157.test.fidl" exclude_regexp="\/\/ (Copyright 20|Use of|found in).*" %}
```