Fixed missing 'break;'s and improved formatting.

[WatcherOfTheSkies]

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• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies:

• All recent builds.

• Issues fixed (if any):

• The intended error message will now be displayed for two cases.

• The formatting is improved to make reading easier.

• n.b. It's not clear what the variables perr and herr really are.

• The error message will mostly append a ':' even if an error is not forthcoming.

• Brief description of code changes (suitable for use as a commit message):

• Fixed missing 'break;'s and improved formatting.

[bmah888]

Yegads that's embarrassing. One of the two problems can trivially be tested like this:

% iperf3 --client localhost --udp --file fubar.dat
iperf3: parameter error - invalid unit value or suffix: '(null)'

Usage: iperf3 [-s|-c host] [options]
Try `iperf3 --help' for more information.

That's the wrong error message. The correct error message (produced with your PR) is actually iperf3: parameter error - cannot transfer file using UDP

Thanks for the PR! Merging shortly.