fix #74: exit when gpg key could not be imported & mention possible workaround in readme

Author: pimlie

README.md

@@ -124,3 +124,6 @@ Kernel v4.8.6 successfully purged
 * sha1sum/sha256sum (to check the .deb checksums)
 * sbsigntool (to sign kernel images for SecureBoot)
 * sudo
+
+## Known issues (with workarounds)
+- GPG is unable to import the key behind a proxy: #74

ubuntu-mainline-kernel.sh

@@ -731,8 +731,8 @@ Optional:
                     log "ok"
                 else
                     logn "failed"
-                    warn "Unable to check signature"
-                    check_signature=0
+                    err "Unable to import ppa key"
+                    exit 1
                 fi
             fi