Misleading error message on bad exec payload from flag

[eyal-mor]

Original Author: @slonokot

Running shore exec with bad JSON payload will produce misleading error message:

$ shore exec -p '{"application": "a", "pipeline": "b",,,}'
required args key 'pipeline' missing
required args key 'application' missing
$ shore exec -p '{"application": "a", "pipeline": "b}'
required args key 'pipeline' missing
required args key 'application' missing
$ shore exec -p '{"application": "a", "pipeline": "b", "parameters":}'
required args key 'pipeline' missing
required args key 'application' missing

The expected behaviour simillar to shore render's one:

"While parsing config: unexpected end of JSON input"