functions: pretty print error messages

Reformat the error messages:
- strip ${CT_LIB_DIR} from scripts path names
- strip ${CT_TOP_DIR} from build.log path and docs path
- overall shorter lines
- point to the known-issues file

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # Compute Alpha-specific values

 CT_DoArchTupleValues () {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}${CT_ARCH_ALPHA_VARIANT}"

 }