web: Remove unwraps from Rust glue

* Remove all unwraps from web/lib.rs.
 * Add convenience methods for grabbing the Ruffle web instance.
   These methods also avoid panics/unwraps when borrowing
   `RefCell`/`Mutex`.
 * Use `warn_on_error` to avoid unwraps from web APIs.

Cargo.lock

@@ -3037,6 +3037,7 @@ dependencies = [
  "ruffle_render_webgl",
  "ruffle_web_common",
  "serde",
+ "thiserror",
  "url",
  "wasm-bindgen",
  "wasm-bindgen-futures",

web/Cargo.toml

@@ -37,6 +37,7 @@ wasm-bindgen-futures = "0.4.19"
 chrono = { version = "0.4", features = ["wasmbind"] }
 getrandom = { version = "0.2", features = ["js"] }
 serde = { version = "1.0.125", features = ["derive"] }
+thiserror = "1.0"
 
 [dependencies.ruffle_core]
 path = "../core"