[LPeg] How can I signal parsing errors with LPeg?

["Soni L." <fakedme@...>]

As seen in http://stackoverflow.com/q/38664815/3691554

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I'm writing an LPeg-based parser. How can I make it so a parsing error 
returns `nil, errmsg`?

I know I can use `error()`, but as far as I know that creates a normal 
error, not `nil, errmsg`.

The code is [pretty 
long](https://bitbucket.org/SoniEx2/mdxml/src/ed38151d8e395134c39c0abaeda84ebfbc5ae278/mdxml.lua), 
but the relevant part is this:

    local eof = lpeg.P(-1)
    local nl = (lpeg.P "\r")^-1 * lpeg.P "\n" + lpeg.P "\\n" + eof -- 
\r for winblows compat
    local nlnoeof = (lpeg.P "\r")^-1 * lpeg.P "\n" + lpeg.P "\\n"
    local ws = lpeg.S(" \t")
    local inlineComment = lpeg.P("`") * (1 - (lpeg.S("`") + nl * nl)) ^ 
0 * lpeg.P("`")
    local wsc = ws + inlineComment -- comments count as whitespace
    local backslashEscaped
    = lpeg.P("\\ ") / " " -- escaped spaces
    + lpeg.P("\\\\") / "\\" -- escaped escape character
    + lpeg.P("\\#") / "#"
    + lpeg.P("\\>") / ">"
    + lpeg.P("\\`") / "`"
    + lpeg.P("\\n") -- \\n newlines count as backslash escaped
    + lpeg.P("\\") * lpeg.P(function(_, i)
        error("Unknown backslash escape at position " .. i) -- this 
error() is what I wanna get rid of.
      end)
    local Line = lpeg.C((wsc + (backslashEscaped + 1 - nl))^0) / 
function(x) return x end * nl * lpeg.Cp()
    local Data = lpeg.S(" \t")^0 * lpeg.Cs((ws / " " + inlineComment / 
"" + backslashEscaped + 1 - (lpeg.S(" \t")^0 * nl))^0) * lpeg.S(" \t")^0 
* nl

----

`Line` and `Data` are the things that when `:match()`ed should return 
`nil, errmsg` on parsing error. Note that only `Line` decides whether a 
line is valid or not, so that's where parsing errors happen. `Data` only 
extracts the data from a (valid) line.

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