Generic handling of Git exit codes, errors and warnings

[mstv]

Current behaviour

The exit code of Git as well as errors and warnings printed to the console by Git are not checked by every GE function.

Expected behaviour

Git command errors are not silently ignored but handled correctly.
Git warnings and errors are displayed clearly, not only as an unhandled exception.

GitCommand shall check the Git exit code and throw a GitFailException unless the calling function indicates that it will handle the exit code itself. The Exception.Message shall contain all Git error messages ("error: ...", "fatal: ...") and all Git warning messages ("warning: ...").

A GitErrorException shall be thrown with all Git error and warning messages as the Exception.Message. These exceptions shall be caught by the calling function or form if applicable and as a fallback by GitModuleForm/GitModuleControl, which shall display the error message in a MessageBox.

GitCommand shall check for Git warning messages and display them in a MessageBox unless the calling function indicates that it will handle them itself.
Though I am not sure whether a MessageBox is really feasible in async and background tasks. If not, a GitWarningException should be thrown with all Git warning messages as the Exception.Message. The drawback of this exception would be that all functions which don't handle the warnings themselves would be canceled.

Git errors and warnings shall be logged in the GitCommandLog.

Environment

• Git Extensions 3.1.0
• Build 3d5da22
• Git 2.21.0.windows.1
• Microsoft Windows NT 6.1.7601 Service Pack 1
• .NET Framework 4.7.2117.0
• DPI 96dpi (no scaling)

[RussKie]

Any thoughts on how this would look like?

[mstv]

With the display of git errors, #7607 wouldn't have failed silently.
After a crash of my PC, Reset all changes failed silently; but at least Reset file or directory changes displayed the good hint below.
I'd prefer a MessageBox with the buttons Ignore and Abort. On Abort, the GitException shall be (re)thrown.

[gerhardol]

A little more info in #8735