fix #5607: schema: say what a valid configuration ID looks like

The 'pve-configid' format is used for many IDs, for example the names
of resource mappings, but an invalid ID only got rejected with:

  invalid configuration ID 'gpu@node1'

which does not tell what is wrong with it, so users had to guess or
look up the regex in the code.

Explain the rules in the error message: an ID must start with a letter,
be at least two characters long and only contain letters, digits, '-'
and '_'. These are the same rules the tooltip of the 'ConfigId'
validation in the web UI lists. Add a test for the message.

Signed-off-by: Michal Fox <me@dualfroz.com>

src/PVE/JSONSchema.pm

@@ -256,7 +256,8 @@ sub pve_verify_configid {
 
     if ($id !~ m/^$CONFIGID_RE\z/) {
         return undef if $noerr;
-        die "invalid configuration ID '$id'\n";
+        die "invalid configuration ID '$id' - it must start with a letter, be at least two"
+            . " characters long and only contain letters, digits, '-' and '_'\n";
     }
     return $id;
 }