Parser throw exception as soon as the parser doesn't find the expected element

[yantang-msft]

For a grammar like below, the expected input is "a b c d e f". And if I the input is like "a b c", the parser will tell me if failed to parse "rule" with the cursor pointing at column 1.
However, it would be better if it tell me it failed to find the "d" in rule2, and point the cursor at column 5.
rule = _ rule1 _ rule2 _ rule3 _ eof
rule1 = _ "a" _ "b" _
rule2 = _ "c" _ "d" _
rule3 = _ "e" _ "f" _
_ = [ \t\r\n]*
eof = !.

A workaround for now would be write the rule like this, but it can explode when the grammar is more complicated.
rule = _ rule1 _ rule2 _ rule3 _ eof
rule1 = _ ("a"/#error{"Expect a"} _ ("b"/#error{"Expect b"} _
...

[yantang-msft]

Just a minor supplement.
If rule2 changed to _ "c" _ ("d")* _, then there can be multiple candidate token after "c". i.e., given the input "a b c", the return the error message is something like "Expect 'd' or 'e' at line 1 column 5".

[otac0n]

I'm combining this feedback with another item in #89.