[fix] Avoid logging Paramiko exceptions for common SSH/network issues

[arafathsiam99]

Avoid logging full stack traces for common SSH connection failures (socket timeout, connection refused, host unreachable) to reduce log noise. These are now handled gracefully without logger.exception().

Checklist

• I have read the OpenWISP Contributing Guidelines.
• I have manually tested the changes proposed in this pull request.
• I have written new test cases for new code and/or updated existing tests for changes to existing code.
• I have updated the documentation.

Reference to Existing Issue

Closes #1000.

Description of Changes

• Introduced better error handling for SSH connection failures in Ssh.exec_command and Ssh.connect methods.
• Avoid logging full exception traces when the error is known (e.g., socket.timeout, socket.error, or known Paramiko connection errors).
• Replaced logger.exception() with logger.warning() or logger.info() as appropriate for expected failures.
• Improved log readability and reduced unnecessary noise from transient or expected network issues.

Screenshot