How to calculate Fst from AMOVA
Error is the estimation that can't be fitted by your parameters (in this case g). 
The amova function in pegas package does the following: 


  This function performs a hierarchical analysis of molecular variance as described in Excoffier et al.
  (1992). This implementation accepts any number of hierarchical levels


And as per the definition of Fixation index, you need the variance in the frequency of the allele between different subpopulations, weighted by the sizes of the subpopulations, the variance of the allelic state in the total population. Which is not provided by amova. Thus you cannot compute the fixation index from amova results.