Fix return value of set_dim

* src/rmatio.c (set_dim): Added return value 0 on success

* src/rmatio.c (read_sparse, read_mat_complex, read_mat_data,
  read_logical): Added check of return value from set_dim
Return 1 on error instead of calling error

* src/rmatio.c (read_mat_char, read_structure_array_with_fields):
  Removed call to error on failure, instead returns 1
Updated documentation

* src/rmatio.c: Updated documentation of argument to functions