synchroniser : use exception instead of return codes

In create_ssh_master_connection return codes where used instead of
proper error handling with exception, replace these codes with the
raising of an appropriate exception.

src/unisync/synchroniser.py

@@ -16,7 +16,7 @@ import logging
 from pathlib import Path
 from typing import cast
 
-from unisync.errors import RemoteMountedError, InvalidMountError
+from unisync.errors import RemoteMountedError, InvalidMountError, UnknownSSHError
 from unisync.config import BackupConfig
 
 logger = logging.getLogger(__name__)
@@ -92,7 +92,7 @@ class Synchroniser:
                 f"Name {backup.backupprefix[:-1]}"
                 ])
     
-    def create_ssh_master_connection(self, control_path:str="~/.ssh/control_%C", connection_timeout:int=60) -> int:
+    def create_ssh_master_connection(self, control_path:str="~/.ssh/control_%C", connection_timeout:int=60) -> None:
         """Creates an ssh master connection.
 
         It is used so the user only has to authenticate once to the remote server.
@@ -108,6 +108,14 @@ class Synchroniser:
         Returns:
             An error code (0 success, 1 TimeoutExpired, 2 KeyboardInterrupt).
             TODO change that to raising the exception.
+
+        Raises:
+            subprocess.TimeoutExpired:
+                The user didn't finish loging in in time.
+            KeyboardInterrupt:
+                The user interrupted the process.
+            UnknownSSHError:
+                An error occured during the connection.
         """
         self.control_path = os.path.expanduser(control_path)
         command = [
@@ -121,16 +129,15 @@ class Synchroniser:
         master_ssh = subprocess.Popen(command)
         try:
             ret_code = master_ssh.wait(timeout=connection_timeout)
-        except subprocess.TimeoutExpired:
+        except subprocess.TimeoutExpired as e:
             print("Time to login expired", file=sys.stderr)
-            return 1
-        except KeyboardInterrupt:
-            return 2
+            raise e
+        except KeyboardInterrupt as e:
+            raise e
 
         if ret_code != 0:
             print("Login to remote failed", file=sys.stderr)
-            return ret_code
-        return 0
+            raise UnknownSSHError
 
 
     def close_ssh_master_connection(self) -> int: