[BUGFIX] Trying to get property of non-object error

During indexing I got a 'Trying to get property of non-object' error
in the NodeIndexer on a line that contained invalid json (a truncated
error message). This change makes sure an error is logged if invalid
json is returned.
Merge pull request #40 from Flowpack/jsonencode

[BUGFIX] Trying to get property of non-object error