chore: Add context to errors thrown on cbor decode

We have the issue in Internet Identity that people complain about hard to
diagnose issues. In particular, we regularly encounter users faced with the
"failed to parse" message thrown out of `borc`. It would be very helpful
to know what the input was that caused that error.

docs/CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Changed
 
 - feat: make `IdbStorage` `get/set` methods generic
+- chore: add context to errors thrown when failing to decode CBOR values.
 
 ## [1.2.0] - 2024-03-25
 

packages/agent/src/cbor.ts

@@ -4,7 +4,7 @@ import borc from 'borc';
 import * as cbor from 'simple-cbor';
 import { CborEncoder, SelfDescribeCborSerializer } from 'simple-cbor';
 import { Principal } from '@dfinity/principal';
-import { concat, fromHex } from './utils/buffer';
+import { concat, fromHex, toHex } from './utils/buffer';
 
 // We are using hansl/simple-cbor for CBOR serialization, to avoid issues with
 // encoding the uint64 values that the HTTP handler of the client expects for
@@ -125,5 +125,9 @@ export function decode<T>(input: ArrayBuffer): T {
     },
   });
 
-  return decoder.decodeFirst(buffer);
+  try {
+    return decoder.decodeFirst(buffer);
+  } catch(e: unknown) {
+    throw new Error(`Failed to decode CBOR: ${e}, input: ${toHex(buffer)}`);
+  }
 }