Update expcetion message in excBadVarName to make it more clear what the problem is

Closed Tomas Dado requested to merge tdado-master-patch-93876 into master

I got confused by this errro:

terminate called after throwing an instance of 'SG::ExcBadVarName'
  what():  SG::ExcBadVarName: Bad name for auxiliary variable `2022_particle'. Variable and class name must be identifiers, and variable name must not be empty.

Then I realised the problem was the first character cannot be a number, but that was not clear from the message.

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