fix: surface deepest error from failed union validation

The "Expected union value" error message is practically useless, so I think it‘s a better DX to surface the deepest validation error (based on property path).

src/errors/errors.ts

@@ -514,13 +514,16 @@ function* FromUndefined(schema: TUndefined, references: TSchema[], path: string,
 }
 function* FromUnion(schema: TUnion, references: TSchema[], path: string, value: any): IterableIterator<ValueError> {
   let count = 0
+  let deepestError: ValueError | undefined
   for (const subschema of schema.anyOf) {
     const errors = [...Visit(subschema, references, path, value)]
     if (errors.length === 0) return // matched
     count += errors.length
+    const pathDepth = (error: ValueError) => error.path.split('/').length
+    deepestError = errors.reduce((deepestError, error) => (!deepestError || pathDepth(error) > pathDepth(deepestError) ? error : deepestError), deepestError)
   }
   if (count > 0) {
-    yield Create(ValueErrorType.Union, schema, path, value)
+    yield deepestError || Create(ValueErrorType.Union, schema, path, value)
   }
 }
 function* FromUint8Array(schema: TUint8Array, references: TSchema[], path: string, value: any): IterableIterator<ValueError> {