fix(core): return proper Error objects from native

[diachedelic]

If I receive news of a Capacitor plugin error via sentry, there are no stack traces because fromNative (in native-bridge.js) throws a deserialized JSON object, not an Error. It's not even clear that the error is from a Capacitor plugin, as it's usually something like { message: "File not found" }.

This pull request transforms the error objects into actual Errors before they are thrown (well, passed to the callback/rejected). Now, at least one can see that the error originated from the native bridge:

Note that this PR uses Object.assign - I'm not sure whether Capacitor supports platforms without it. This is the MDN compatibility table:

[diachedelic]

I figured Object.assign not being supported by Android WebView was probably a problem, so I replaced it with an old school solution.

[josh-m-sharpe]

If this is true - big 👍

[jcesarmobile]

Thanks! it's really a bug, web and electron return proper errors, but Android and iOS weren't.