[xfa] Dump JS errors to console in debug mode

This CL writes any JS exceptions to the console in XFA if running in
Debug mode. This makes it possible to see when an error happens in JS
execution.

Bug: pdfium:1097
Change-Id: Ida14050328c5e6e85ab2704bb5dddfec370dddf1
Reviewed-on: https://pdfium-review.googlesource.com/33154
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: Ryan Harrison <rharrison@chromium.org>

fxjs/cfxjse_context.cpp

diff --git a/fxjs/cfxjse_context.cpp b/fxjs/cfxjse_context.cpp
index 03bcc4d..6f2b770 100644
--- a/fxjs/cfxjse_context.cpp
+++ b/fxjs/cfxjse_context.cpp
@@ -293,9 +293,16 @@
       ASSERT(!trycatch.HasCaught());
       if (lpRetValue)
         lpRetValue->m_hValue.Reset(m_pIsolate, hValue);
+
       return true;
     }
   }
+
+#ifndef NDEBUG
+  v8::String::Utf8Value error(GetIsolate(), trycatch.Exception());
+  fprintf(stderr, "JS Error: %ls\n", WideString::FromUTF8(*error).c_str());
+#endif  // NDEBUG
+
   if (lpRetValue) {
     lpRetValue->m_hValue.Reset(m_pIsolate,
                                CreateReturnValue(m_pIsolate, trycatch));