cranelift-module: make backend error an anyhow::Error

This allows us to retain richer information from backend errors.
We already have `anyhow` as a dep in several places in the wasmtime
tree, and in cranelift-faerie. faerie is the only user of this
variant.

Existing code that puts a String into the Backend error can trivially
adapt their code to emit an anyhow::Error.

cranelift/module/src/module.rs

@@ -149,7 +149,7 @@ pub enum ModuleError {
     Compilation(#[from] CodegenError),
     /// Wraps a generic error from a backend
     #[error("Backend error: {0}")]
-    Backend(String),
+    Backend(#[source] anyhow::Error),
 }
 
 /// A convenient alias for a `Result` that uses `ModuleError` as the error type.