Incorrect error messages during compilation

[cyberthirst]

Version Information

• vyper Version (output of vyper --version): commit 4b4e188

Issue description

• In VariableDecl.validate(), instead of calling the method
  self._pretty_location() to pretty-print the location of the given
  variable, self._pretty_location is used.
• In _CreateBase.build_IR, context.check_is_not_constant should be
  called with an f-string and not a regular string to have self._id
  as the id of the built-in in the error message.

credits: @trocher