Improve error message for `[[<-`

[GegznaV]

This is cbeleites/hyperSpec#240

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Currently, if [[<- is used, this error is thrown:

flu[[, 3]] <- NA_real_

Error in .local(x, i, j, ..., value = value) :
The spectra matrix may only be indexed by i (spectra) and l (wavelengths). j (data column) must be missing.

Improve the message by:

• providing a more exact place where the error occurred (i.e., not .local(...) is not informative enough for users).
• add an example in the message, how to fix (e.g. instead of, e.g., x[[, 20]] <- 1, use x[[, , 20]] <- 1)