Ignore unreadable directories when re-generating the class autoloader cache (closes #3194)

Use the CATCH_GET_CHILD flag when constructing a
RecursiveIteratorIterator, to ignore exceptions thrown by the
RecursiveDirectoryIterator when a directory is not readable.
Merge pull request #589 from eguaj/3.2-A3194

Ignore unreadable directories when re-generating the class autoloader cache (closes #3194)