make usage of foo is a user-defined error more informative

[timotheecour]

/cc @Araq

before PR

procs marked with {.error.} give very uninformative/cryptic errors when used.

Example:

var a: seq[int]
echo a.isNil

t10464.nim(11, 8) Error: usage of 'isNil' is a user-defined error

totally cryptic, and we have no way to tell where this error comes from, especially in more complex cases where the type of a is generic, and there are many overloads.

after PR

we show where the symbol is defined, eg:
t10464.nim(11, 8) Error: usage of 'isNil' is an {.error.} defined at /Users/timothee/git_clone/nim/Nim_prs/lib/system.nim(1509, 1)

now it's clear: looking up the definition there shows:

proc isNil*[T](x: seq[T]): bool {.noSideEffect, magic: "IsNil", nilError.}

(and looking up {.nilError.} shows this resolves to {.error.})