tour: handle error rather than print both

[crisman]

In "tour/methods/20" ("Exercise: Errors") just after the slide explaining "calling code should handle errors by testing whether the error equals nil" the example code in the errors exercise provides a main that calls functions expecting error values and does not check them. Checking error values is the expected go way of doing things and should be shown. In the successful case it is also odd to show the "nil" of no error when printing the correct answer (see golang/tour#273). Printing the likely undefined float64 root value when there is an error is also confusing.

Update "methods/exercise-errors.go" and matched solution to only print error on error and only root otherwise.

Fixes golang/tour#273 Fixes golang/tour#230

[gopherbot]

This PR (HEAD: 625960b11fa6fd9fe3fcda1d1c25339f31749082) has been imported to Gerrit for code review.

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