Don't use argument as callback if it's not a valid callback function.
Throw a valid exception instead explaining the issue.
Adds to #7070 ("DNS — Throw meaningful error(s)").
getServers returns an array of ips that are currently being used for
resolution
setServers takes an array of ips that are to be used for resolution,
this will throw if there's invalid input but preserve the original
configuration
A typo in the variable name makes it throw a ReferenceError instead of
the expected "Unknown type" error when dns.resolve() is passed a bad
record type argument.
Fixes the following exception:
ReferenceError: type is not defined
at Object.exports.resolve (dns.js:189:40)
at /Users/bnoordhuis/src/master/test/simple/test-c-ares.js:48:9
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