Only turn warnings into errors where needed

warnings.filterwarnings('error') applied to the whole program, so
any warning raised by a library (e.g. a DeprecationWarning) would
crash it. Limit it to the ssh connection (unknown host key) and
the password prompt (no terminal available).

simple_backup/simple_backup.py

@@ -37,8 +37,6 @@ from glob import glob
 
 from dotenv import load_dotenv
 
-warnings.filterwarnings('error')
-
 try:
     import paramiko
     from paramiko import RSAKey, Ed25519Key, ECDSAKey, DSSKey
@@ -346,7 +344,10 @@ class Backup:
         ssh.set_missing_host_key_policy(paramiko.WarningPolicy())
 
         try:
-            ssh.connect(self.ssh_host, username=self.ssh_user)
+            # WarningPolicy emits a UserWarning for unknown host keys
+            with warnings.catch_warnings():
+                warnings.simplefilter('error', UserWarning)
+                ssh.connect(self.ssh_host, username=self.ssh_user)
 
             return ssh
         except UserWarning:
@@ -374,8 +375,12 @@ class Backup:
 
         if self.ssh_keyfile is None:
             try:
-                password = getpass(
-                    f'{self.ssh_user}@{self.ssh_host}\'s password: ')
+                # Fail instead of reading the password with echo if no terminal is available
+                with warnings.catch_warnings():
+                    warnings.simplefilter('error', GetPassWarning)
+                    password = getpass(
+                        f'{self.ssh_user}@{self.ssh_host}\'s password: ')
+
                 ssh.connect(self.ssh_host, username=self.ssh_user,
                             password=password)