Check for IO errors when loading binary shader

[ljbade]

This fixes #740

I had to add checks for IO errors (I thought iostream threw on errors).

I also put in the sanity check code from android-mason for good measure.

[ljbade]

I just found out that you can actually enable C++ exceptions with http://en.cppreference.com/w/cpp/io/basic_ios/exceptions

@jfirebaugh Would this be a better way to handle errors?

[kkaefer]

Can we use our caching infrastructure instead of writing directly to disk? E.g., we could introduce a cache:// scheme that only reads/writes to the cache, but doesn't attempt to load anything from disk. If it can't find the URL in cache, it just returns a not found error and the requesting application can generate the resource manually and then store it into the cache.

[ljbade]

I suppose we could do that. Does the caching infrastructure support overwriting?

Since binary shader depends on GL state combinations, loading different maps/areas could trigger more information to be included in the binary shader each time the app is closed.

[ljbade]

@kkaefer Perhaps I should cut a new ticket for that? In the mean time this pull request will at least fix a crash that was holding me up on working on another bug.

[jfirebaugh]

Yeah, I generally prefer exceptions to manual error checking. In this case, I think the code should also handle any other exceptions (e.g. from MBGL_CHECK_ERROR) by abandoning the cached shaders, so it's probably easiest if the whole block is surrounded with try { } catch (...) { }.

[ljbade]

@jfirebaugh converted to exceptions

Also fixed the missing call to map::terminate which was causing havok with shader destructor. I think this might have been causing some other crashes on Android too.

Can you review and merge if all is OK?

[ljbade]

Also fix rare crash when HTTP request finishes after Map termination begins.

[jfirebaugh]

Which std::remove is this? I'm surprised a one-argument call compiles.