Replaced use of RuntimeError exception in pypjua with custom PyPJUAError and PJSIPError exception classes

darcs-hash:20090114131447-58103-c500ab42b1b07b1c2860935f05efb98ce2e6ef4b
Added core.error.pxi file

darcs-hash:20090114143040-58103-72b817f1f2a9acda9366935af269bd484f8ec5ef