The error message should be printf style, not just concatenated. The
current message is garbled if the command or result contains things that
look like formatting directives:

`Internal error occurred: error sending request: Post "https://10.10.10.102:10250/exec/default/volume-test/volume-test?command=sh&command=-c&command=echo+local-path-test+%!!(MISSING)E(MISSING)+%!!(MISSING)F(MISSING)data%!!(MISSING)F(MISSING)test&error=1&output=1": proxy error from 127.0.0.1:6443 while dialing 10.10.10.102:10250, code 502: 502 Bad Gateway`
Signed-off-by: Brad Davidson <brad.davidson@rancher.com>
(cherry picked from commit 45195e26)
Signed-off-by: Brad Davidson <brad.davidson@rancher.com>