cli: Do not stop on JSON trace parsing failure
cli: Do not stop on JSON trace parsing failure

Sometimes Docker API returns invalid JSON with traces
on Docker for Mac.

This leads to exception:

/Library/Ruby/Gems/2.0.0/gems/json-2.0.2/lib/json/common.rb:156:in
`parse': 743: unexpected token at
'{"stream":"Step 2 : LABEL re-build-environment true (Excon::Error::Socket)
      ' (JSON::ParserError)

Signed-off-by: Dmitry Fleytman <a8cf97adadec4e1b734a39bc5aea71b5741cfca1@daynix.com>

rbld/rebuild,rbld/rebuild,daynix/rebuild,daynix/rebuild,daynix/rebuild,rbld/rebuild