Improvement: Standardized all exception messages in SiteContainers.php

What does this PR do?

• Improves and standardizes exception/help text messages in SiteContainers.php (or whichever file you changed).
• Adds relevant context (site path, site URL) to error/help messages.
• Removes inconsistent leading spaces and vague wording.

Why?

• Previous messages were inconsistent and sometimes unclear.
• Better messages help users and developers debug faster.

Testing

• Ran php -l for syntax check.
• Verified help text/output locally.