If you look into the implementation of ASSERT, you will find that it actually cannot output error message when the condition is failed.

Apply the CORRADE_ASSERT shipped with Corrade library instead.

In this PR:
-) replaced all the ASSERT with CORRADE_ASSERT;
-) added "Error: " to some of the error messages;

-) replaced "Error" with class::functionName at the beginning of every error message;
-) used CORRADE_ASSERT_UNREACHABLE;