target/xtensa: extra debug info for "xtensa exe" failures

- Read and display EXCCAUSE on exe error
- Clean up error messages
- Clarify "xtensa exe" documentation

Signed-off-by: ianst <ianst@cadence.com>
Change-Id: I90ed39f6afb6543c0c873301501435384b4dccbe
Reviewed-on: https://review.openocd.org/c/openocd/+/7982


Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>