Add help message when diff fails

[runewake2]

Related to #1378

This introduces an additional error message when a potential error is detected during a pkg diff (error codes non-0 or 1 - diff returning error code 1 indicates a difference was detected).

Kpt uses an external diff tool to compare packages using the provided diff strategy. Some of these tools may not support 3 way diffs, others may not support diffing over directories. In order to attempt to improve the experience of working with kpt pkg diff these new errors are added to attempt to help provide guidance around resolving the error.

For example this changes the behavior of a 3way diff on clients where diff does not support 3 directories from

➜  kpt pkg diff package@main --diff-type=3way
/usr/bin/diff: extra operand `/tmp/kpt-234176370/target-main'
/usr/bin/diff: Try `/usr/bin/diff --help' for more information.

error: exit status 2
exit status 1

to

➜  kpt pkg diff package@main --diff-type=3way
/usr/bin/diff: extra operand `/tmp/kpt-234176370/target-main'
/usr/bin/diff: Try `/usr/bin/diff --help' for more information.

The selected diff tool (/usr/bin/diff) exited with an error. It may not support the chosen diff type (3way). To use a different diff tool please provide the tool using the --diff-tool flag. 

For more information about using kpt's diff command please see the commands --help.
error: exit status 2
exit status 1