feat: add Error field and improve logging

Add Error field to GolangCILintResult struct for better error handling.
Improve logging by:
- Adding Debugf method to logger interface
- Adding early return after logging JSON marshal errors
- Replacing DebugJSON with Debugf for improved command visibility
- Standardizing printf formatting with explicit newlines

Author: nametake

golangci-lint.go

@@ -51,5 +51,6 @@ type GolangCILintResult struct {
 			Enabled          bool   `json:"Enabled"`
 			EnabledByDefault bool   `json:"EnabledByDefault,omitempty"`
 		} `json:"Linters"`
+		Error string `json:"Error"`
 	} `json:"Report"`
 }

handler.go

@@ -64,7 +64,7 @@ func (h *langHandler) lint(uri DocumentURI) ([]Diagnostic, error) {
 	} else {
 		cmd.Dir = dir
 	}
-	h.logger.DebugJSON("golangci-lint-langserver: golingci-lint cmd", cmd)
+	h.logger.Debugf("golangci-lint-langserver: golingci-lint cmd: %v\n", cmd)
 
 	b, err := cmd.Output()
 	if err == nil {
@@ -134,7 +134,7 @@ func (h *langHandler) linter() {
 
 		diagnostics, err := h.lint(uri)
 		if err != nil {
-			h.logger.Printf("%s", err)
+			h.logger.Printf("%s\n", err)
 
 			continue
 		}
@@ -146,7 +146,7 @@ func (h *langHandler) linter() {
 				URI:         uri,
 				Diagnostics: diagnostics,
 			}); err != nil {
-			h.logger.Printf("%s", err)
+			h.logger.Printf("%s\n", err)
 		}
 	}
 }

logger.go

@@ -10,6 +10,7 @@ var _ logger = (*stdLogger)(nil)
 
 type logger interface {
 	Printf(format string, args ...interface{})
+	Debugf(format string, args ...interface{})
 	DebugJSON(label string, arg interface{})
 }
 
@@ -29,6 +30,10 @@ func (l *stdLogger) Printf(format string, args ...interface{}) {
 	l.stderr.Printf(format, args...)
 }
 
+func (l *stdLogger) Debugf(format string, args ...interface{}) {
+	l.stderr.Printf(format, args...)
+}
+
 func (l *stdLogger) DebugJSON(label string, arg interface{}) {
 	if !l.debug {
 		return
@@ -37,6 +42,7 @@ func (l *stdLogger) DebugJSON(label string, arg interface{}) {
 	b, err := json.Marshal(arg)
 	if err != nil {
 		l.stderr.Println(err)
+		return
 	}
 
 	l.stderr.Println(label, string(b))