Draft: Master: change pur_mm_purple_connect()

This commit changes pur_mm_purple_connect() from a void to a gboolean. This is required in pur_mm_state in the case of PUR_MM_STATE_MODEM_FOUND, as if pur_mm_get_modem_state fails, the execution follows through and purple-mm-sms falsely connects the modem when it should not (e.g. if the modem has no messaging capabilities).

If you wish to test this, please comment out this line:

https://source.puri.sm/Librem5/purple-mm-sms/-/blob/master/mm-sms.c#L741