This tweaks the construction of the error message when using `expect`/`runCmd`. With
this change, the stdout/stderr is placed after the message "Command '<command>' did not
return successfully".
Before:
```
AssertionError: False is not True : Command 'p whatever
Error output:
error: <some error message>
' did not return successfully
```
After:
```
AssertionError: False is not True : Command 'p whatever' did not return successfully
Error output:
error: <some error message>
```
bfe2994560