When contents generation fails, don't bail out

This replaces `panic` which aborts aptly execution with warning
message on console. So aptly continues publishing actions, but
`Contents` indexes might be incomplete.

Error will be printed every time contents generation is triggered.

system/t06_publish/PublishRepo29Test_gold

@@ -0,0 +1,14 @@
+Loading packages...
+Generating metadata files and linking package files...
+[!] Failed to generate package contents:  unable to read .tar archive from ${HOME}/.aptly/pool/3a/0a/libboost-broken-program-options-dev_1.49.0.1_i386.deb: unexpected EOF
+Finalizing metadata files...
+Signing file 'Release' with gpg, please enter your passphrase when prompted:
+Clearsigning file 'Release' with gpg, please enter your passphrase when prompted:
+
+Local repo local-repo has been successfully published.
+Please setup your webserver to serve directory '${HOME}/.aptly/public' with autoindexing.
+Now you can add following line to apt sources:
+  deb http://your-server/ maverick main
+Don't forget to add your GPG key to apt with apt-key.
+
+You can also use `aptly serve` to publish your repositories over HTTP quickly.

system/t06_publish/repo.py

@@ -694,3 +694,15 @@ class PublishRepo28Test(BaseTest):
         self.check_not_exists('public/dists/maverick/main/Contents-i386.gz')
         self.check_exists('public/dists/maverick/main/debian-installer/binary-i386/Release')
         self.check_not_exists('public/dists/maverick/main/Contents-udeb-i386.gz')
+
+
+class PublishRepo29Test(BaseTest):
+    """
+    publish repo: broken .deb file for contents
+    """
+    fixtureCmds = [
+        "aptly repo create local-repo",
+        "aptly repo add local-repo ${testfiles}",
+    ]
+    runCmd = "aptly publish repo -keyring=${files}/aptly.pub -secret-keyring=${files}/aptly.sec -distribution=maverick local-repo"
+    gold_processor = BaseTest.expand_environ