Signed-off-by: Philippe Blain <[email protected]>

In git-mergetool--lib.sh::get_merge_tool_path, we check if the chosen
tool is valid via valid_tool and exit with an error message if not. This
error message mentions "Unknown merge tool", even if the command the
user tried was 'git difftool --tool=unknown'. Use the global 'TOOL_MODE'
variable for a more correct error message.

Signed-off-by: Philippe Blain <[email protected]>

In git-mergetool--lib.sh::setup_tool, we check if the given tool is a
known builtin tool, a known variant, or a user-defined tool by calling
setup_user_tool, and we return with the exit code from setup_user_tool
if it was called. setup_user_tool checks if {diff,merge}tool.$tool.cmd
is set and quietly returns with an error if not.

This leads to the following invocation quietly failing:

	git mergetool --tool=unknown

which is not very user-friendly. Adjust setup_tool to output an error
message before returning if setup_user_tool returned with an error.

Note that we do not check the result of the second call to
setup_user_tool in setup_tool, as this call is only meant to allow users
to redefine 'cmd' for a builtin tool; it is not an error if they have
not done so.

Note that this behaviour of quietly failing is a regression dating back
to de8dafb (mergetool: break setup_tool out into separate
initialization function, 2021-02-09), as before this commit an unknown
mergetool would be diagnosed in get_merge_tool_path when called from
run_merge_tool.

Signed-off-by: Philippe Blain <[email protected]>

In setup_tool, we check if the given tool is a known variant of a tool,
and quietly return with an error if not. This leads to the following
invocation quietly failing:

	git mergetool --tool=vimdiff4

Add an error message before returning in this case.

Signed-off-by: Philippe Blain <[email protected]>

Since the introduction of 'initialize_merge_tool' in de8dafb
(mergetool: break setup_tool out into separate initialization function,
2021-02-09), any errors from this function are ignored in
git-difftool--helper.sh::launch_merge_tool, which is not the case for
its call in git-mergetool.sh::merge_file.

Despite the in-code comment, initialize_merge_tool (via its call to
setup_tool) does different checks than run_merge_tool, so it makes sense
to abort early if it encounters errors. Add exit calls if
initialize_merge_tool fails.

Signed-off-by: Philippe Blain <[email protected]>