SshSupport#runSshCommand: don't throw exception in finally block

SshSupport#runSshCommand: don't throw exception in finally block

The CommandFailedException which was thrown in finally block is silently
discarded [1]. Refactor this method to throw the exception after the
finally block.

This fixes the warning "Null comparison always yields false: The
variable failure can only be null at this location".

[1] https://wiki.sei.cmu.edu/confluence/display/java/ERR04-J.+Do+not+complete+abruptly+from+a+finally+block
https://docs.oracle.com/javase/specs/jls/se8/html/jls-14.html#jls-14.20.2

Change-Id: Idbfc303d9c9046ab9a43e0d4c6d65d325bdaf0ed
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>