Improve connection error description.

[hsorby]

The current description of an error in the connection element does not specify which connection it came from, for example currently we might have the following connection error description:

"Connection in model 'starwars' specifies 'Luke' as variable_1 but the corresponding component_1 is invalid."

But I think we should also have the name of the connection present in this message so the above error message could be improved to the following:

"Connection 'return_of_the_jedi' in model 'starwars' specifies 'Luke' as variable_1 but the corresponding component_1 is invalid."

[nickerso]

connections don't have names, but the id could be used if specified, otherwise the two component names in addition to the two variable names is usually enough to narrow things down to a manageable set.