Fix documentation of qa-catalogue script

Only one command can be passed. Use option -a to executed multiple
commands.

README.md

@@ -245,11 +245,11 @@ If you do not want to
 #### run
 
 ```bash
-catalogues/[your script] [command(s)]
+catalogues/[your script] [command]
 ```
 or
 ```bash
-./qa-catalogue --params="[options]" [command(s)]
+./qa-catalogue --params="[options]" [command]
 ```
 
 The following commands are supported:

qa-catalogue

@@ -3,7 +3,7 @@ set -ueo pipefail
 
 usage() {
   cat << EOF
-Usage: $0 [options] <command[,command...]>
+Usage: $0 [options] <command>
 
 QA catalogue for analysing library data
 
@@ -27,6 +27,11 @@ EOF
   exit
 }
 
+error() {
+  echo $@ >&2
+  exit 1
+}
+
 # optionally load default values
 [ -f ./setdir.sh ] && . ./setdir.sh
 
@@ -98,11 +103,9 @@ EOF
 # don't proceed if web configuration shall be updated but cannot do so
 if [[ ! -z "$WEB_CONFIG" ]]; then
   if [[ ! -f "$WEB_CONFIG" ]]; then
-    echo "web configuration file missing: $WEB_CONFIG" >&2
-    exit 1
+    error "web configuration file missing: $WEB_CONFIG"
   elif [[ -z "$CATALOGUE" ]]; then
-    echo "catalogue short name not set" >&2
-    exit 1
+    error "catalogue short name not set"
  fi
 fi