Update: improve return values used in c_base_utf8 class functions (and related)

Update: improve return values used in c_base_utf8 class functions (and related)

I believe I was returning TRUE/FALSE types out of habit and for consistency.
For string processing functions, it makes far more sense to always return a c_base_return_string.
This simplifies the code the caller has to write.

The caller can still identify errors using the c_base_return::s_has_error() function.
The caller need only to call ->get_value_exact() if they do not care whether an error occured.
This will always return a string (and on error, it returns an empty string).