Follow-up to PR #690 to check chrome_pref before hacking

[kensoh]

No description provided.

[kensoh]

@siowyisheng for your review and testing. After reviewing I found its better to add if exist to check that the file exists before trying to hack. Otherwise, maybe for some systems, the code will throw error twice due to the file not found in that standard location.

This change would let the hack work if the file is found, which should be the case for most users. But for users which do not have that file, avoid hacking and throw error messages.

[kensoh]

@siowyisheng added another fine-tuning. On systems without write access to the files (lack of privilege or maybe other Chrome processes still running) will show an Access denied error. This commit mutes this error message from TagUI output as it's plain misleading to users what to do about it.

[siowyisheng]

tested, works. merging.