utils/check-package: handle missing files

When running check-package before completing commits for a change, if
any files are setup for removal, check-package will throw
FileNotFoundError exceptions instead of generating a warning state. For
example:

 $ utils/docker-run make check-package
 Traceback (most recent call last):
   ...
 FileNotFoundError: [Errno 2] No such file or directory: 'package/.../0001-some-removed-patch.patch'
 make: *** [Makefile:1264: check-package] Error 1

This commit will now catch FileNotFoundError and populate a warning
message:

 $ utils/docker-run make check-package
 package/.../0001-some-removed-patch.patch: missing; unstaged file removal?
 package/.../0002-another-removed-patch.patch: missing; unstaged file removal?
 427843 lines processed
 3 warnings generated
 make: *** [Makefile:1264: check-package] Error 1

Signed-off-by: James Knight <git@jdknight.me>
Reviewed-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Julien Olivain <ju.o@free.fr>

utils/check-package

@@ -257,18 +257,23 @@ def check_file_using_lib(fname):
         nwarnings += warn
 
     lastline = ""
-    with open(fname, "r", errors="surrogateescape") as f:
-        for lineno, text in enumerate(f):
-            nlines += 1
-            for name, cf in objects:
-                if cf.disable.search(lastline):
-                    continue
-                line_sts = cf.check_line(lineno + 1, text)
-                warn, fail = print_warnings(line_sts, name in xfail)
-                if fail > 0:
-                    failed.add(name)
-                nwarnings += warn
-            lastline = text
+    try:
+        with open(fname, "r", errors="surrogateescape") as f:
+            for lineno, text in enumerate(f):
+                nlines += 1
+                for name, cf in objects:
+                    if cf.disable.search(lastline):
+                        continue
+                    line_sts = cf.check_line(lineno + 1, text)
+                    warn, fail = print_warnings(line_sts, name in xfail)
+                    if fail > 0:
+                        failed.add(name)
+                    nwarnings += warn
+                lastline = text
+    except FileNotFoundError:
+        print(f"{fname}: missing; unstaged file removal?")
+        nwarnings += 1
+        return nwarnings, nlines
 
     for name, cf in objects:
         warn, fail = print_warnings(cf.after(), name in xfail)