Error reporting: check existence of the path to siva files

[bzz]

If example from our README https://github.com/src-d/spark-api#pyspark-api-usage is tried literally

from sourced.spark import API as SparkAPI
from pyspark.sql import SparkSession
 
spark = SparkSession.builder.appName("test").master("local[*]").getOrCreate()
api = SparkAPI(spark, '/path/to/siva/files')
api.repositories.filter("id = 'github.com/mawag/faq-xiyoulinux'").references.filter("name = 'refs/heads/HEAD'").show()

with the )non-existent path to .siva files_, it will result in

---------------------------------------------------------------------------
Py4JJavaError                             Traceback (most recent call last)
<ipython-input-1-7f67465f882f> in <module>()
      4 spark = SparkSession.builder.appName("test").master("local[*]").getOrCreate()
      5 api = SparkAPI(spark, '/path/to/siva/files')
----> 6 api.repositories.filter("id = 'github.com/mawag/faq-xiyoulinux'").references.filter("name = 'refs/heads/HEAD'").show()

/usr/local/spark/python/pyspark/sql/dataframe.py in show(self, n, truncate)
    334         """
    335         if isinstance(truncate, bool) and truncate:
--> 336             print(self._jdf.showString(n, 20))
    337         else:
    338             print(self._jdf.showString(n, int(truncate)))

/usr/local/spark/python/lib/py4j-0.10.4-src.zip/py4j/java_gateway.py in __call__(self, *args)
   1131         answer = self.gateway_client.send_command(command)
   1132         return_value = get_return_value(
-> 1133             answer, self.gateway_client, self.target_id, self.name)
   1134 
   1135         for temp_arg in temp_args:

/usr/local/spark/python/pyspark/sql/utils.py in deco(*a, **kw)
     61     def deco(*a, **kw):
     62         try:
---> 63             return f(*a, **kw)
     64         except py4j.protocol.Py4JJavaError as e:
     65             s = e.java_exception.toString()

/usr/local/spark/python/lib/py4j-0.10.4-src.zip/py4j/protocol.py in get_return_value(answer, gateway_client, target_id, name)
    317                 raise Py4JJavaError(
    318                     "An error occurred while calling {0}{1}{2}.\n".
--> 319                     format(target_id, ".", name), value)
    320             else:
    321                 raise Py4JError(

Py4JJavaError: An error occurred while calling o39.showString.
: org.apache.spark.sql.catalyst.errors.package$TreeNodeException: execute, tree:
Exchange hashpartitioning(repository_id#11, 200)
+- *Filter (((isnotnull(name#12) && (name#12 = refs/heads/HEAD)) && isnotnull(repository_id#11)) && (repository_id#11 = github.com/mawag/faq-xiyoulinux))
   +- *Scan GitRelation(org.apache.spark.sql.SQLContext@4faae818,references,/path/to/siva/files,/tmp) [repository_id#11,name#12,hash#13] PushedFilters: [IsNotNull(name), EqualTo(name,refs/heads/HEAD), IsNotNull(repository_id), EqualTo(repository_id,..., ReadSchema: struct<repository_id:string,name:string,hash:string>

which is not very clean error message.

This can be fixed by implementing a proper check and error reporting, if the given path does not exist, in the Scala part.