[web] verbose invalid path exception (#480)
[web] verbose invalid path exception (#480)

This commit outputs more exception information when the path is invalid.
openthread/borderrouter,bukepo/borderrouter,jwhui/borderrouter,openthread/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix,openthread/ot-br-posix,bukepo/borderrouter,openthread/ot-br-posix,openthread/borderrouter,bukepo/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix,openthread/borderrouter,jwhui/borderrouter,jwhui/borderrouter,jwhui/borderrouter,bukepo/borderrouter,openthread/borderrouter,bukepo/borderrouter,openthread/borderrouter,jwhui/borderrouter