Internally name all patterns for log parsing flexibility

closes #1436

This also fixes the bad behavior of waiting until runtime to return log
parsing pattern compile errors when a pattern was simply unfound.

closes #1418

Also protect against user error when the telegraf user does not have
permission to open the provided file. We will now error and exit in this
case, rather than silently waiting to get permission to open it.

plugins/inputs/tail/tail.go

@@ -86,9 +86,10 @@ func (t *Tail) Start(acc telegraf.Accumulator) error {
 		for file, _ := range g.Match() {
 			tailer, err := tail.TailFile(file,
 				tail.Config{
-					ReOpen:   true,
-					Follow:   true,
-					Location: &seek,
+					ReOpen:    true,
+					Follow:    true,
+					Location:  &seek,
+					MustExist: true,
 				})
 			if err != nil {
 				errS += err.Error() + " "