Add serde_html_form to Error; use serde_core for greater abstraction.

Signed-off-by: Jason Volk <jason@zemos.net>

src/core/Cargo.toml

@@ -85,6 +85,8 @@ reqwest.workspace = true
 ring.workspace = true
 ruma.workspace = true
 sanitize-filename.workspace = true
+serde_core.workspace = true
+serde_html_form.workspace = true
 serde_json.workspace = true
 serde_regex.workspace = true
 serde_yaml.workspace = true