dlite-validate output confusing information

[francescalb]

dlite-validate returns the full traceback, and thus makes it confusing to understand the validation.

Currently this is the output I get when validating a datamodel with wrong input for dimensions:

dlite-validate vasp.yaml
Warning: Behavior singleInterpreter is not configured. It will be enabled by default from v0.7.0. See https://sintef.github.io/dlite/user_guide/configure_behavior_changes.html for more info.
DLiteOtherError: calling load() in Python plugin 'yaml'
To see error messages from Python storages, please rerun with the
DLITE_PYDEBUG environment variable set.
For example: export DLITE_PYDEBUG=

: TypeError: dimensions must be either a sequence or a mapping

Only the last line should be shown as the immediate understanding of the above error is that there is an error in the yaml plugin (not that the yaml plugin raises and error).