Print coefficient string in error message when string not found in DOLFIN wrappers

[Prof Garth Wells]

It would be helpful if the message

dolfin::dolfin_error("generated code for class Form",
                     "access coefficient data",
                     "Invalid coefficient");

would print the coefficient string.

Fix is not super straightforward because the code block is re-used for the int version of virtual std::size_t coefficient_number(const std::string& name) const