libtuv will fail on unknown error

[konradlipner]

I am refering to https://github.com/Samsung/libtuv/blob/master/src/uv-common.c#L118

On TizenRT there are some hard to find error definitions. This line cause core dump, while it could return "unknown error" / NULL. Current logic seems to be debug purpose, and end user would not expect exception because there is no proper string defined. It is not easy to get rid of this, and require recompilation/flashing of whole OS.