Reduce duplication in error messages (#532)

* Reduce duplication in error messages

This commit removes duplication in error messages where the same text
would show up multiple times in a fully rendered error message.

When using `derive(Error)` when the `#[from]` attribute is used there's
no need to also render that payload into the error string because the
`#[from]` establishes a "backtrace" which means that when the full
context of an error is rendered it will include the `#[from]` in the
lower frames of the backtrace anyway.

This commit audits the `derive(Error)` implementations to avoid
duplication in the rendered error messages, ensuring that if `#[from]`
is used then the `#[from]` field isn't also rendered in the textual
description.

* Search the full error in wast assertions

Don't just search the top error, but search the whole backtrace by using
the `{:?}` format instead of `{}`.

crates/jit/src/context.rs

@@ -23,10 +23,10 @@ pub struct UnknownInstance {
 #[derive(Error, Debug)]
 pub enum ContextError {
     /// An unknown instance name was used.
-    #[error("{0}")]
+    #[error("An error occured due to an unknown instance being specified")]
     Instance(#[from] UnknownInstance),
     /// An error occured while performing an action.
-    #[error("{0}")]
+    #[error("An error occurred while performing an action")]
     Action(#[from] ActionError),
 }