can mxPath error include the path?

Use traceback(). The problem is that if we always include the stack trace, then we'll get the complaint "OMG what is all this error output, it's so confusing." And the stacktrace is all or nothing, there's no such thing as a partial stacktrace.

> traceback()
6: stop(paste("mxPath() call will generate", numPaths, "paths but you have specified", 
       len, lenName), call. = FALSE)
5: pathCheckSingleLength(numPaths, length(values), "values")
4: pathCheckLengths(from, to, arrows, values, free, labels, lbound, 
       ubound)
3: generatePath(from, to, all, arrows, values, free, labels, lbound, 
       ubound)
2: mxPath(from = c("x", "y"), arrows = 2, free = TRUE, values = c(1, 
       0.1, 1), labels = c("Varx", "Vary"))
1: mxModel("Def model", type = "RAM", mxData(myData, type = "raw"), 
       manifestVars = c("x", "y"), latentVars = "DefDummy", mxPath(from = c("x", 
           "y"), arrows = 2, free = TRUE, values = c(1, 0.1, 1), 
           labels = c("Varx", "Vary")), mxPath(from = "x", to = "y", 
           arrows = 2, free = TRUE, values = c(0.1), labels = c("Covxy")), 
       mxPath(from = "one", to = c("x", "y", "DefDummy"), arrows = 1, 
           free = c(TRUE, TRUE, FALSE), values = c(1, 1, 1), labels = c("meanx", 
               "meany", "data.def")), mxPath(from = "DefDummy", 
           to = c("x", "y"), arrows = 1, free = c(TRUE, TRUE), values = c(1, 
               1), labels = c("beta_1", "beta_2")))

As a compromise the error message has been modified to print:

Error: mxPath() call will generate 2 paths but you have specified 3 values with 'from' argument assigned to 'x' and 'y' and 'to' argument assigned to 'NA'