Improve BLOSC_ERROR_NULL macro

[oscargm98]

• It would be interesting to modify the BLOSC_ERROR_NULL macro so that it gives more descriptive error messages in this way:
  #define BLOSC_ERROR_NULL(pointer, rc)
  do {
  if (pointer == NULL) {
  BLOSC_TRACE_ERROR("Pointer %s is NULL", #pointer "");
  return rc;
  }
  } while (0)

It will actually print out the name of the pointer and should keep strings to a minimum.

• This pattern also exists a lot in the code:
  if (rc < 0) {
  return rc;
  }

So adding a helper for that might be nice too.
And possibly rename BLOSC_ERROR_NULL to match.

#define BLOSC_RETURN_NOTOK(rc)
do {
if (rc < 0) {
return rc;
}
} while (0)

#define BLOSC_ERROR_NULL(pointer, rc)
do {
if (pointer == NULL) {
BLOSC_TRACE_ERROR("Pointer %s is NULL", #pointer "");
return rc;
}
} while (0)

[DimitriPapadopoulos]

BLOSC_ERROR_NULL might be called on variables with the same name, for example:

  BLOSC_ERROR_NULL(context, NULL);

Perhaps, file name (__FILE__) and line (__LINE__) would be more useful