Exceptions during hg push should be errors

[rmunn]

The IProgress implementations have an ErrorEncountered flag that is set when their WriteException or WriteError methods are called. But PushToTarget was only calling WriteMessageWithColor, so if the only exception during a Send/Receive was happening during the hg push step, then ErrorEncountered was incorrectly still false.

Fixes #366.

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This change is 

[rmunn]

Note that this might change the output of any code that uses SIL.Windows.Forms.Progress.LogBox as the progress dialog, because LogBox writes a full exception stacktrace in its WriteException implementation, rather than just the exception message as the existing code was doing. If this matters, a possible fix would be WriteError(err.Message) instead of WriteException, but that would potentially lose useful debugging info that users might copy and paste into an email. So I chose to use WriteException here, but I'll change it if requested.

[github-actions]

Test Results

       8 files  ±0     333 suites  ±0   2h 34m 24s ⏱️ + 11m 23s
   988 tests ±0     932 ✔️ ±0    56 💤 ±0  0 ❌ ±0 
3 145 runs  ±0  3 022 ✔️ ±0  123 💤 ±0  0 ❌ ±0 

Results for commit 0925bb5. ± Comparison against base commit 1ab24b9.