Issue: Some failed tests causes the cx-playground to crash

[kenje4090]

To recreate the issue:

1. Move cx-playground/examples/to_be_added_examples/concatenating-strings.cx to cx-playground/examples
2. run cx-playground
3. On the web app, run concatenating-strings.
   This would make the web app crash instead of showing the error.

Note: some examples under cx-playground/examples might already run properly. Please check other examples. And try to create examples that will crash and see if cx-playground will crash too or not.

TODO:
Solution 1:

• Try to improve cx-playground, so the issue will be solved without touching skycoin/cx. If this is not possible, try solution 2.

Solution 2:

• CX-Playground is dependent on CX.
• Create a new branch based on develop branch of skycoin/cx
• Improve the error handling(panics) of skycoin/cx so that the cx-playground will not crash if there are errors or panics in the CX program that is ran in it.
• Create also a branch in skycoin/cx-playground that has imported the branch with improved error handling so when we use this branch in cx-playground, it will not crash even if we run the tests inside cx-playground-examples/to_be_added_examples.