Verifies that a file exists in HDFS.
The name of a file in HDFS. The file name can include a path that is either absolute or relative to the current path.
If this function returns
TRUE, then you can attach the data and use it in a
hadoop.run function. You can also use this function to validate an HDFS identifier and ensure that the data exists.
TRUE if the identifier is valid and the data exists, or
FALSE if the object is not found