Write a function calc(str) that evaluates an expression in Reverse Polish Notation (RPN). RPN is an expression where operands and operators are written in a special order:

• - Operands (numbers) and operators (+, -, *, /) are separated by spaces.
• - An operation is performed on the two most recent numbers encountered before the operator.
• - The expression is evaluated from left to right.

If the input string is incorrect, the function should return an error message. Possible errors:

• "Error in Syntax" — if the expression contains a syntax error.
• "Error in Operands" — if an operand is not a number.
• "Division by zero" — if division by zero occurs in the expression.