throw error for incorrect number of factors in extractBhatt

[goldingn]

When factor is of the wrong length extractBhatt should throw a useful error.

[goldingn]

Also throw a more helpful error if NAs appear in GAUL codes produced by getGAUL (i.e. if user didn't check rasters had same mask)

[goldingn]

Or if occurrence didn't contain the GAUL column