toml: add excerpt to value parse errors

[Larpon]

Before:

OK   [32/189] "/home/user/v/vlib/toml/tests/testdata/burntsushi/toml-test/tests/invalid/integer/double-sign-plus.toml"...
     toml.parser.Parser.value value expected .boolean, .quoted, .lsbr, .lcbr or .number got "plus" "+"

After:

OK   [32/189] "/home/user/v/vlib/toml/tests/testdata/burntsushi/toml-test/tests/invalid/integer/double-sign-plus.toml"...
     toml.parser.Parser.value value expected .boolean, .quoted, .lsbr, .lcbr or .number got "plus" "+" in this (excerpt): "...n-plus = ++99\n..."

(My plan, btw - is to polish the error excerpts to be more like V's sometime in the future)