QR Code Scanner Fixes

[hanwencheng]

fix #247

mainly 3 fixes:

a15259d: fix a scanning user UX issue, after unavailable QR code, only trigger the scan when the user clicks the button.

b5e239f: add address detected warning.

eaf9f52: remove an unhandled promise rejection warning, code should return when error detected.

[pmespresso]

Suggested change

	for (var n = 0; n < hex.length; n += 2) {
	for (let n = 0; n < hex.length; n += 2) {

[Tbaut]

Thanks a lot for the PR.
Unrelated side note: We really need to add a linter to this project. The ton of nits and whitespace modifications totally dilute the real changes or the PR. I'll give it a play on my android now.

[hanwencheng]

@Tbaut I agree, before I make the commit, I just do prettier --write with the file. It would be nice if we have a lint standard and to integrate the lint into CI test.

[Tbaut]

👍 Works well. I tested with Ethereum and Substrate addresses.
Signing still works (tested on Ethereum)

[pmespresso]

works well on iOS! thank you!