Better error message for missing Lepton in subColvar

[jhenin]

When using the NAMD regtest example of NeuralNetwork in a non-Lepton enabled build of VMD (and I assume other tools) I get:

colvars:   Error while constructing colvar number 1 : deleting.
    Error: the CV  expects one or more nesting components.
  Error: in setting up component "neuralNetwork".
Error parsing configuration file

whereas when used at the main colvar level, there is a more informative error message. The same error message should be shown when in a subColvar.

[HanatoK]

Maybe I need to remove the ifdef guards in https://github.com/Colvars/colvars/blob/789029131bca128ff36017d4bdd0d98aaec495aa/src/colvar.cpp#L933-L935

[jhenin]

I think that's right!