Replace throw with assert if -fno-exceptions

[GIldons]

On systems with exception disabled current code is not able to compile.
Replace throw with assert in such systems.

[jimemo]

This problem exists when compiling in VSCode/PlatformIO.
Workaround:

1. Edit cpp-base64\base64.cpp, within function pos_of_char()
   Comment out:
   throw std::runtime_error("Input is not valid base64-encoded data.");
   Add:
   return 0;
2. Delete cpp-base64\test.cpp and cpp-base64\test-google.cpp

Apart from that it works great.