fix(cache): remove entry from _pending in case of exception

[IgorKhramtsov]

Issue

If an exception is thrown during loader execution - future with error result will be stuck in the _pending map and it will be returned every time putIfAbsent is called. There is no way to evict it either, since evict method only affects _cache map.
The exception in loader may be thrown in case of network error when SvgNetworkLoader is used.

Change

This change adds a catchError handler to remove future from the _pending map in case of failure. Also a regression test is added that will fail without this fix.

[zanderso]

Hi there, The Flutter team is now maintaining this package in the flutter/packages repository at this location. We'd appreciate it if you could direct this PR to that location instead. Thanks!