Early exit in error callback from Wasm (#9)

* Early exit in error callback from Wasm

This avoids Wasm execution hanging due to problems such as wrong public
input.

Mimics circom_runtime behaviour with less detailed debug information.

See https://github.com/iden3/circom_runtime/blob/master/js/witness_calculator.js#L52-L64

Adds test for wrong public input. Without early exit, the test stalls.
With it, the Circom build step fails as expected.

* chore: clean up error handling

* ci: add caching

Co-authored-by: Georgios Konstantopoulos <me@gakonst.com>

.github/workflows/ci.yml

@@ -1,6 +1,6 @@
 on:
   push:
-    branches: 
+    branches:
       - master
   pull_request:
 
@@ -35,6 +35,10 @@ jobs:
             export PATH=$HOME/bin:$PATH
             solc --version
 
+      - uses: Swatinem/rust-cache@v1
+        with:
+          cache-on-failure: true
+
       - name: cargo test
         run: |
             export PATH=$HOME/bin:$PATH