Fix error when using checksum method and no file exists for a source glob

Closes #131

internal/status/glob.go

@@ -19,7 +19,7 @@ func glob(dir string, globs []string) (files []string, err error) {
 		}
 		f, err := zglob.Glob(g)
 		if err != nil {
-			return nil, err
+			continue
 		}
 		files = append(files, f...)
 	}

testdata/checksum/Taskfile.yml

@@ -2,6 +2,7 @@ build:
   cmds:
     - cp ./source.txt ./generated.txt
   sources:
+    - ./**/glob-with-inexistent-file.txt
     - ./source.txt
   generates:
     - ./generated.txt