Handle callback error in readAsTextAsync & check CRC

[kevincharm]

Hi, I found some issues around readAsTextAsync:
readAsTextAsync passes a callback down to getDataAsync but only checks for a non-empty data object:

adm-zip/adm-zip.js

Line 116 in 3f00a03

item.getDataAsync(function (data) {

Then getDataAsync passes down that callback to decompress:

adm-zip/zipEntry.js

Line 245 in 3f00a03

getDataAsync : function(/*Function*/callback, pass) {

Then decompress can invoke that callback with non-empty (!) data and an error string:

adm-zip/zipEntry.js

Line 62 in 3f00a03

if (async && callback) callback(data, Utils.Errors.BAD_CRC);//si added error

So this PR just handles that potential error that can propagate up. Then I also added some tests around zips with good/bad CRCs. Also found that CRC check wasn't failing on zips with deliberately bad corrupted CRCs.