prevent crash when disk cache file is corrupted

We have received several crash reports related to TMDiskCache like below:


	Application Specific Information:
	*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[NSKeyedUnarchiver initForReadingWithData:]: incomprehensible archive (0x62, 0x70, 0x6c, 0x69, 0x73, 0x74, 0x30, 0x30)'
	
	Last Exception Backtrace:
	0 CoreFoundation 0x3377b3e7 0x336b9000 + 795623
	1 libobjc.A.dylib 0x3b5fb963 0x3b5f3000 + 35171
	2 CoreFoundation 0x3377b307 0x336b9000 + 795399
	3 Foundation 0x340139d1 0x33fe2000 + 203217
	4 Foundation 0x340749df 0x33fe2000 + 600543
	5 XXXX 0x00ac8183 __34-[TMDiskCache objectForKey:block:]_block_invoke (in XXXX) (TMDiskCache.m:416)
	6 libdispatch.dylib 0x3ba15793 0x3ba14000 + 6035
	7 libdispatch.dylib 0x3ba18b3b 0x3ba14000 + 19259
	8 libdispatch.dylib 0x3ba1667d 0x3ba14000 + 9853
	9 libdispatch.dylib 0x3ba19613 0x3ba14000 + 22035
	10 libdispatch.dylib 0x3ba197d9 0x3ba14000 + 22489
	11 libsystem_c.dylib 0x3ba3d7f1 0x3ba37000 + 26609
	12 libsystem_c.dylib 0x3ba3d684 0x3ba37000 + 26244
	
It is because the users' cache file is somehow corrupted and It seems the line crashed `object = [NSKeyedUnarchiver unarchiveObjectWithFile:[fileURL path]];` needs a try catch block to prevent crash when this happens.