Show more user-friendly track loader errors

[danielmitterdorfer]

With this commit we add more details when Rally cannot load a track. In
all cases we include the path to the completely rendered track file. We
also detect syntax errors separately and provide the specific context in
which the error happens.

[danielmitterdorfer]

Here is an example of the new output for a missing comma:

[ERROR] Cannot list. Could not load '/Users/daniel/.rally/benchmarks/tracks/default/geonames/track.json': Expecting ',' delimiter: line 46 column 7 (char 1066). Lines containing the error:

    {
      "name": "default",
      "operation-type": "search"
      "body": {
------^ Error is here
        "query": {
          "match_all": {}

The complete track has been written to '/var/folders/pg/gbmwwwms3n50fttdmnqd4sl40000gn/T/tmpy3b8nifs.json' for diagnosis.

another example with a missing double-quote at the end of a string:

[ERROR] Cannot list. Could not load '/Users/daniel/.rally/benchmarks/tracks/default/geonames/track.json': Invalid control character at: line 45 column 33 (char 1059). Lines containing the error:

    },
    {
      "name": "default",
      "operation-type": "search,
--------------------------------^ Error is here
      "body": {
        "query": {

The complete track has been written to '/var/folders/pg/gbmwwwms3n50fttdmnqd4sl40000gn/T/tmpf33rckho.json' for diagnosis.

[dliappis]

Very useful, thanks! LGTM