Rename error trace to the more user friendly Counterexample

[FedericoPonzi]

Not sure if it's worth it because from docs and code we refer to this section as Error Trace.

[lemmy]

Renaming “Error Trace” to “Counterexample” also aids in distinguishing “Counterexamples” from specific errors (which cause TLC to throw an error or exception).

[hwayne]

I think this is a good idea if we can make it distinguish between property/assertion violations and TLC errors, like adding a string to an integer. Basically, if this could still be "error trace":

[lemmy]

To me, the error trace, called stack trace elsewhere, is the call chain of expressions that led to the error. The error trace is not an object in the theory of TLA or TLA+.