consolidate error messages

[jakubmisek]

Consolidate throwing of error and warning messages such as invalid arguments, invalid arguments type, etc. Sometimes we throw an exception, sometimes we output a warning. Everything should be handled by PhpException object, and possibly extensible.

• [ ] re-design PhpException class to handle common scenarios
• [ ] review library functions and parameter handling - use PhpException helper class for everything
• [ ] extensible PhpException
• [ ] warning message should contain the name of the library function causing the problem