Debuginfo

Mainly replaces 
```
if (condition...) return CL_SOME_ERROR;
```
and its "goto" variant with macros that do the same plus print a message.
Needs enabling at compilation & then at runtime.
Documented in  ```doc/sphinx/source/env_variables.rst```

Also adds a bunch of checks that were missing in various API calls (Not completely all required checks though).

Also adds a bunch of methods to pocl_util.c and pocl_image_util.c to be used by the checks.
Please use only /* */ comments in C codes for now. Is this an intended comment?