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>

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