Return 1 if the package name in rospack plugins can not be found

[esteve]

@hugomatic and I found out that if a user passes an invalid package name to rospack plugins, the error code is still 0:

$ rospack plugins --attrib=plugin pluginliba; echo $?
[rospack] Warning: no such package pluginliba
0

being analogous to ls:

$ ls invalid_path ; echo $?
ls: cannot access invalid_path: No such file or directory
2

This PR changes the behavior to return 1:

$ rospack plugins --attrib=plugin pluginliba; echo $?
[rospack] Warning: no such package pluginliba
1

[dirk-thomas]

Should the message then be change to "error" and go to stderr?

[esteve]

It already goes to stderr, but I can change the code to call logError if you like.

[dirk-thomas]

Since its not a warning anymore (rc != 0) please change it to logError.

[esteve]

Fixed.

[dirk-thomas]

lgtm