Warning instead of error if clang-tidy and cppcheck are not installed?

[aminya]

I think we should enable clang-tidy and cppcheck by default, but instead of throwing errors if they are missing, we can give warnings.

The use case is that I want to enable clang-tidy and cppcheck on my own machine because I am sure they are installed, but I want to also allow the users to build the project even if they do not have these.

We can still allow this to error out if WARNING_AS_ERRORS is enabled.