show meaningfull error message when MDSPLUS_MODELS_PATH is not set

[prasad-sawantdesai]

No description provided.

[prasad-sawantdesai]

Closes #46

[SimonPinches]

lgtm

[prasad-sawantdesai]

from imas_core import _al_lowlevel as al
from imas_core import imasdef

al.imas_core_config_enable_exceptions()
try:
    status, de_ctx = al.al_begin_dataentry_action("imas:mdsplus?path=./testdb_create", imasdef.FORCE_CREATE_PULSE)
    print(f"Created pulse OK, de_ctx={de_ctx}")
    al.al_close_pulse(de_ctx, imasdef.CLOSE_PULSE)
except Exception as exc:
    print(f"ERROR: {exc}")

$ unset MDSPLUS_MODELS_PATH
]$ python test_mdsplus_create.py
ERROR: b'al_begin_dataentry_action: [ALBackendException = MDSPLUS_MODELS_PATH is not set. Set it to the MDSplus models directory (e.g. export MDSPLUS_MODELS_PATH=/path/to/models/mdsplus) before creating a pulse file.]'
Error status=-3