get_variable does not throw exception or error in case does not find the variable.

[MarkWendler]

mcpState = x2c_scope.get_variable("mcpState")
mcpState.get_value()
Traceback (most recent call last):
File "", line 1, in
AttributeError: 'NoneType' object has no attribute 'get_value'

Looks like mcpState variable is not found (Actually I did not load elf file...)

[edras]

The behavior is to log the error.
What is changed now is to differentiate between different cases:

1. If the variable does not exist in elf file, we log message "Variable '{name} not found!" and return None
2. If the variable is found, but an error occured when parsing, we log the error message and return None

#96