Refuse an exclude pattern that does not compile at config load (closes #455)

An exclude pattern that did not compile was skipped with a warning during the backup, which then stored what the user meant to leave out. Config.Validate now refuses it, naming the setting and quoting the pattern, for example exclude[2]: "build[" is not a valid exclude pattern: ...

The compile step moves to a new package, internal/exclude, as exclude.Compile; Validate and the scanner both call it. The scanner's skip-and-warn path is gone; a pattern that does not compile there now fails the scanner.

Model: opus-5-5

internal/config/exclude.go

@@ -0,0 +1,39 @@
+package config
+
+import (
+	"fmt"
+
+	"sneak.berlin/go/vaultik/internal/exclude"
+)
+
+// validateExcludes refuses an exclude pattern, global or in a snapshot,
+// that does not compile, naming the setting and quoting the pattern.
+func (c *Config) validateExcludes() error {
+	err := validateExcludeList("exclude", c.Exclude)
+	if err != nil {
+		return err
+	}
+
+	for _, name := range c.SnapshotNames() {
+		err = validateExcludeList("snapshots."+name+".exclude",
+			c.Snapshots[name].Exclude)
+		if err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+// validateExcludeList refuses the first pattern in patterns that does not
+// compile. setting is the name of the list, as in "exclude".
+func validateExcludeList(setting string, patterns []string) error {
+	for i, pattern := range patterns {
+		_, _, err := exclude.Compile(pattern)
+		if err != nil {
+			return fmt.Errorf("%s[%d]: %w", setting, i, err)
+		}
+	}
+
+	return nil
+}