Improve exception handling of APIs in classLoader.cpp

Some APIs in classLoader.cpp have a TRAPS declaration, which means they can throw an exception under error conditions:

E.g.,

ClassPathEntry* ClassLoader::create_class_path_entry(const char *path, const struct stat* st,
                                                     bool throw_exception,
                                                     bool is_boot_append,
                                                     bool from_class_path_attr,
                                                     TRAPS) {

But this function can also return NULL to indicate an error (if throw_exception==false). This makes the callers look rather awkward:

    ClassPathEntry* new_entry = NULL;
    new_entry = create_class_path_entry(path, &st, /*throw_exception=*/true, is_boot_append, from_class_path_attr, CHECK_false);
    if (new_entry == NULL) {
      return false;
    }

To improve readability, we should add new functions like:

- create_class_path_entry_or_fail(...TRAPS) - throw exception on error (never return NULL on success)
- create_class_path_entry_or_null - return NULL on error (never throw exceptions)

Also, there are APIs like ClassPathEntry::open_stream() that are declared with TRAPS but the implementations never throw exceptions.