Misleading error message on bad exec payload from flag

Original Author: @slonokot

Running `shore exec` with bad JSON payload will produce misleading error message:
```shell
$ 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"`

Start with the `shore exec -p` payload parsing path and compare it with `shore render`'s error handling. Reproduce the malformed JSON examples and make `shore exec` report the JSON parsing error instead of missing required keys; done when the examples produce the expected parsing message.

Written by the indexing model from the issue text.