Fixed how ShellCommand show exceptions.

Prior to this change, it was duplicating the exception. Example:

$ adb shell cmd car_service create-user --flags 4444 --timeout 100 --hal-only Dude

Exception occurred while executing: new user flags mismatch on existing users on {.requestId = 2, .newUserInfo = {.userId = 11, .flags = 0x115c}, .newUserName = Dude, .usersInfo = {.currentUser = {.userId = 11, .flags = NONE}, .numberUsers = 3, .existingUsers = [{.userId = 0, .flags = 0x9}, {.userId = 10, .flags = ADMIN}, {.userId = 11, .flags = NONE}]}}
java.lang.IllegalArgumentException: new user flags mismatch on existing users on {.requestId = 2, .newUserInfo = {.userId = 11, .flags = 0x115c}, .newUserName = Dude, .usersInfo = {.currentUser = {.userId = 11, .flags = NONE}, .numberUsers = 3, .existingUsers = [{.userId = 0, .flags = 0x9}, {.userId = 10, .flags = ADMIN}, {.userId = 11, .flags = NONE}]}}
	at com.android.internal.util.Preconditions.checkArgument(Preconditions.java:69)
	at android.car.userlib.UserHalHelper.toVehiclePropValue(UserHalHelper.java:503)
	at com.android.car.hal.UserHalService.createUser(UserHalService.java:339)

Now it's showing the failed command instead:

Exception occurred while executing 'create-user --flags 4444 --timeout 100 --hal-only Dude':
java.lang.IllegalArgumentException: new user flags mismatch on existing users on {.requestId = 2, .newUserInfo = {.userId = 11, .flags = 0x115c}, .newUserName = Dude, .usersInfo = {.currentUser = {.userId = 11, .flags = NONE}, .numberUsers = 3, .existingUsers = [{.userId = 0, .flags = 0x9}, {.userId = 10, .flags = ADMIN}, {.userId = 11, .flags = NONE}]}}
	at com.android.internal.util.Preconditions.checkArgument(Preconditions.java:69)
	at android.car.userlib.UserHalHelper.toVehiclePropValue(UserHalHelper.java:503)
	at com.android.car.hal.UserHalService.createUser(UserHalService.java:339)

Test: manual verification
Bug: 150408921

Change-Id: I42221fbe2410770b0755c32d16620acc71d8d5d2