[wasm][asm.js] Don't squish error location because module end not reached.

We recently started emitting an error because a module's validation
doesn't reach the end (due to lack of exports).
This contained a mistake that prevents many most other types
of errors from actually getting out.

Ensuring export errors (or not reaching then end) doesn't clobber
other errors.

TODO(bradnelson): Figure out a way to structure tests so this gets
caught.

BUG=v8:4203
R=gdeepti@chromium.org

Review-Url: https://codereview.chromium.org/2632593002
Cr-Commit-Position: refs/heads/master@{#42301}

src/asmjs/asm-wasm-builder.cc

@@ -123,7 +123,7 @@ class AsmWasmBuilderImpl final : public AstVisitor<AsmWasmBuilderImpl> {
     if (HasStackOverflow()) {
       return false;
     }
-    if (!typer_finished_) {
+    if (!typer_finished_ && !typer_failed_) {
       typer_->FailWithMessage("Module missing export section.");
       typer_failed_ = true;
     }