Return errors applying filters as render errors

[thessem]

When a Filter returns an error as its second argument it generates a panic with the error.

I have changed this behaviour so that the error is returned from the RenderAndPass function.

For example, if there is a filter called toTitle that returns "test error" as its error, the error returned will be 'Liquid error: error applying filter "toTitle" ("test error") in {{ "test message" | toTitle }}'.

Apologies if panicing in this case is the expected behaviour and this was done intentionally, I can always just catch the panic when I call the Render step, I just think this way is more elegant.

[coveralls]

Coverage decreased (-0.2%) to 87.694% when pulling 8ee8cef on Proximaio:feature/filter_errors_returned_as_errors into de5fffa on osteele:master.

[coveralls]

Coverage decreased (-0.04%) to 87.907% when pulling 3d99b41 on Proximaio:feature/filter_errors_returned_as_errors into de5fffa on osteele:master.

[osteele]

Yes, this looks good. Thanks!