[BUG] If you don't have admin rights, the installing the test certificate silently fails

[sma]

When running dart run msix:create in the terminal without admin rights, the installation silently fails and display "successfully installed" which took me (as a windows noob) really long to find out. An error message would have been really helpful. Right now, all output from the spawned process is ignored.

Even better would have been if the install script would automatically try to acquire admin rights.

At least, a bold notice in the README would be useful, I think.