TT#29620 Update ngcp-api.inc to follow Sipwise shell style

Also removed unnecessary exit code handling through the separate variable.
Fixed error output to STDERR and add one more check for the file readability.

Change-Id: I38f54f027f16c45cd54f69a6bd42b3911fc2eec6

tools/ngcp-api.inc

@@ -1,15 +1,23 @@
+#!/bin/bash
+
+set -e
+
 importcreds () {
-    if [ -f "$CREDS" ]; then
-        echo "$CREDS present, checking perms"
-        set +e; stat "$CREDS" | grep 'Access: (0600' 1>/dev/null; r=$?; set -e
-        if [ "$r" = "0" ]; then
-            echo "$CREDS permissions ok"
-            . "$CREDS"
-        else
-            echo "$CREDS must have permissions 600"
-            exit 1
-        fi
-    fi
-}
+  if [ ! -f "${CREDS}" ]; then
+    return
+  fi
 
+  if [ ! -r "${CREDS}" ]; then
+    echo "Error: cannot read ${CREDS}" >&2
+    exit 1
+  fi
 
+  echo "${CREDS} present, checking perms"
+  if stat "${CREDS}" | grep -q 'Access: (0600' 1>/dev/null ; then
+    echo "${CREDS} permissions ok"
+    . "${CREDS}"
+  else
+    echo "Error: ${CREDS} must have permissions 600" >&2
+    exit 1
+  fi
+}