Improve "missing symbol" error messages (ROOT-6755).
Improve "missing symbol" error messages (ROOT-6755).

marsupial/cling,karies/cling,root-mirror/cling,root-mirror/cling,marsupial/cling,karies/cling,karies/cling,marsupial/cling,marsupial/cling,root-mirror/cling,perovic/cling,marsupial/cling,root-mirror/cling,marsupial/cling,root-mirror/cling,karies/cling,karies/cling,perovic/cling,perovic/cling,karies/cling,perovic/cling,root-mirror/cling,perovic/cling