Extract quota exceeded testing to use an array

[Flygenring]

This improves readability while making it simpler to maintain and extend

[pamelafox]

Hmmm. Maybe this is me being overly curmudgeonly, but I believe that Array.indexOf is a more recent addition to browsers than localStorage, so introducing it could potentially reduce the usability of lscache across browsers. I'm hesitant to make a change that could reduce the browser compatibility, if it doesn't have a big benefit. Anyone else have thoughts?

[peter-gribanov]

About browsers support localStorage and Array.indexOf.
The main problem as always in IE.

According to StatCounter:

• IE8 uses 0.38%
• IE9 uses 0.25%.

According to NetMarketShare:

• IE6 uses 0.18%
• IE7 uses 0.16%
• IE8 uses 1.06%
• IE9 uses 0.46%

In my opinion, this optimization does not add readability and performance.
I would recommend to postpone this PR for some time.