LibGfx/WebP: Redo error handling

Most places used to call `context.error()` to report an error,
which would set the context's state to `Error` and then return an
`Error::from_string_literal()`.

This is somewhat elegant, but it doesn't work: Some functions this
code calls returns ErrorOr<>s that aren't created by `context.error()`,
and for these we wouldn't enter the error state.

Instead, manually check error-ness at the leaf entry functions of the
class:

1. Add a set_error() helper for functions returning bool
2. In the two functions returning ErrorOr<>, awkwardly check the error
   manually.  If this becomes a very common pattern, maybe we can add
   a `TRY_WITH_HANDLER(expr, error_lambda)` which would invoke a
   lambda on error. We could use that here to set the error code.

No real behavior change (except we enter the error state more often
when something goes wrong).

Userland/Libraries/LibGfx/ImageFormats/WebPLoader.h

@@ -31,6 +31,8 @@ public:
     virtual ErrorOr<Optional<ReadonlyBytes>> icc_data() override;
 
 private:
+    bool set_error(ErrorOr<void>&&);
+
     WebPImageDecoderPlugin(ReadonlyBytes, OwnPtr<WebPLoadingContext>);
 
     OwnPtr<WebPLoadingContext> m_context;