Serialize errors before invoking postMessage

[Rob--W]

Sometimes (when an error occurs), PDF.js gets stuck. The viewer is then indefinitely waiting for a promise to resolve, which never happens because of the DataCloneError. This PR addresses the issue by converting the error to a string before throwing.

Firefox: "DataCloneError: The object could not be cloned."
Chrome: "DataCloneError: Failed to execute 'postMessage' on 'WorkerGlobalScope': An object could not be cloned."

Here's an isolated test case, if you want to see what happens when an Error object is passed to postMessage:

try {
    window.postMessage(new Error(), '*')
} catch (e) {
    alert(e);
}

[Hengjie]

I've noticed this issue as well, it's great to hear there's a fix for it!

[Snuffleupagus]

/botio test

[pdfjsbot]

From: Bot.io (Windows)

---

Received

Command cmd_test from @Snuffleupagus received. Current queue size: 0

Live output at: http://107.22.172.223:8877/393d6ae51d091d9/output.txt

[pdfjsbot]

From: Bot.io (Linux)

---

Received

Command cmd_test from @Snuffleupagus received. Current queue size: 0

Live output at: http://107.21.233.14:8877/7a3d8aafe6e2564/output.txt

[pdfjsbot]

From: Bot.io (Linux)

---

Success

Full output at http://107.21.233.14:8877/7a3d8aafe6e2564/output.txt

Total script time: 19.20 mins

• Font tests: Passed
• Unit tests: Passed
• Regression tests: Passed

[pdfjsbot]

From: Bot.io (Windows)

---

Success

Full output at http://107.22.172.223:8877/393d6ae51d091d9/output.txt

Total script time: 19.61 mins

• Font tests: Passed
• Unit tests: Passed
• Regression tests: Passed

[Rob--W]

@timvandermeij On IRC you asked for a PDF that triggers the error. Take any of the examples from https://encrypted.google.com/search?q=%22DataCloneError%3A%22+pdf.js, e.g. the PDF from https://bugzilla.mozilla.org/show_bug.cgi?id=911761 (https://bug911761.bmoattachments.org/attachment.cgi?id=798520).

(in PDF.js: 1.1.132)

[Snuffleupagus]

Looks good, thanks for the patch!