fix: improve error message for invalid slot names

[imatiach-msft]

fix: improve error message for invalid slot names

resolves
#887

related LightGBM code for special characters:
https://github.com/microsoft/LightGBM/blob/master/include/LightGBM/utils/common.h#L965

[imatiach-msft]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[codecov]

Codecov Report

Merging #897 into master will increase coverage by 1.84%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #897      +/-   ##
==========================================
+ Coverage   83.31%   85.16%   +1.84%     
==========================================
  Files         189      189              
  Lines        8769     8783      +14     
  Branches      558      548      -10     
==========================================
+ Hits         7306     7480     +174     
+ Misses       1463     1303     -160     

Impacted Files	Coverage Δ
...com/microsoft/ml/spark/lightgbm/LightGBMBase.scala	92.50% <100.00%> (+1.59%)	⬆️
...osoft/ml/spark/io/http/PartitionConsolidator.scala	68.88% <0.00%> (-26.67%)	⬇️
.../microsoft/ml/spark/vw/VowpalWabbitRegressor.scala	70.00% <0.00%> (+10.00%)	⬆️
...a/com/microsoft/ml/spark/vw/VowpalWabbitBase.scala	89.04% <0.00%> (+15.06%)	⬆️
...a/com/microsoft/ml/spark/vw/HasSumcollisions.scala	100.00% <0.00%> (+25.00%)	⬆️
.../scala/com/microsoft/ml/spark/vw/VectorUtils.scala	96.42% <0.00%> (+28.57%)	⬆️
...n/scala/com/microsoft/ml/spark/vw/HasNumBits.scala	85.71% <0.00%> (+28.57%)	⬆️
...microsoft/ml/spark/vw/VowpalWabbitFeaturizer.scala	87.50% <0.00%> (+35.22%)	⬆️
...oft/ml/spark/vw/VowpalWabbitMurmurWithPrefix.scala	70.73% <0.00%> (+36.58%)	⬆️
...microsoft/ml/spark/vw/VowpalWabbitClassifier.scala	90.90% <0.00%> (+39.39%)	⬆️
... and 7 more

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[imatiach-msft]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[imatiach-msft]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).