On the brew-it tab (with the date), if you enter FG > OG, the program terminates with an assert

[matty0ung]

If you enter an FG that is greater than the OG, you get ASSERT: "og >= fg" in file ../src/Algorithms.cpp, line 458 and the program terminates.

We should just warn the user instead of terminating the program!

I discovered this whilst (unsuccessfully so far) trying to reproduce #730