Better error reporting

[albertolerda]

Errors are quite difficult to understand, for example in this case the script is

Rule unknown ignore
Given I connect to 'metadata' and do get and output into 'metadata_res'

Given I have a 'float' named 'result' in 'metadata res'
Given I rename 'result' to 'result metadata'
Then print data

and there is no data or keys, the error is a connect is required. In this case, there is one statement and the data is simple so one can track it easily, but it can become problematic with multiple statements.

What about including in the error reporting:

1. which statement thrown the error
2. specify the name of the data you are trying to read. For connect, there is just one, but it could be useful in the send.

We could also use the JSON format for error reporting to provide consistency in the output (same format in case of error and success).