Performance Optimization: Replace errmsg with error struct.

[soasme]

Given tables.go,

Before:

$ time ./cli ast --grammar-file ../tests/golang-v1.17.peg --grammar-entry SourceFile ../tables.go > /dev/null

real	0m4.781s
user	0m3.879s
sys	0m0.022s

After:

$ time ./cli ast --grammar-file ../tests/golang-v1.17.peg --grammar-entry SourceFile ../tables.go > /dev/null

real	0m1.253s
user	0m0.843s
sys	0m0.018s

Conclusion:

The change will cut down the execution time by 78%.