Fix two issues found by static analysis

I ran the Linter static analyzer on scalding and these two issues are IMO worthy of fixing:

    [warn] scalding/scalding-core/src/main/scala/com/twitter/scalding/TuplePacker.scala:72: Unless there are side-effects, .filter(...).headOption can be replaced by .find(...).
    [warn]     val missing = Dsl.asList(fields).filter { f => !getSetters.contains(f.toString) }.headOption
    [warn]                                                                                       ^

    [warn] scalding/scalding-core/src/main/scala/com/twitter/scalding/Source.scala:108: This string format will throw: java.util.UnknownFormatConversionException: Conversion = '%'
    [warn]     assert(!sources.containsKey(srcName), "Source %s had collision in uuid: %".format(this, uuid))
    [warn]                                           ^