Use clear and consistent error output

Previously, it was not always clear that the error messages were such. Adding a prefix to the feedback.Error*() output
ensures this will be clear and that the error messages will follow a consistent format.

Author: per1234

Diff for: check/checkdata/library.go

@@ -62,7 +62,7 @@ func InitializeForLibrary(project project.Type, schemasPath *paths.Path) {
 		url := "http://downloads.arduino.cc/libraries/library_index.json"
 		httpResponse, err := http.Get(url)
 		if err != nil {
-			feedback.Errorf("%s Unable to download Library Manager index from %s", err, url)
+			feedback.Errorf("Unable to download Library Manager index from %s: %s", err, url)
 			os.Exit(1)
 		}
 		defer httpResponse.Body.Close()

Diff for: command/command.go

@@ -33,7 +33,7 @@ import (
 // ArduinoCheck is the root command function.
 func ArduinoCheck(rootCommand *cobra.Command, cliArguments []string) {
 	if err := configuration.Initialize(rootCommand.Flags(), cliArguments); err != nil {
-		feedback.Errorf("Configuration error: %v", err)
+		feedback.Errorf("Invalid configuration: %v", err)
 		os.Exit(1)
 	}
 

Diff for: configuration/configuration.go

@@ -107,7 +107,7 @@ func Initialize(flags *pflag.FlagSet, projectPaths []string) error {
 			targetPath := paths.New(projectPath)
 			targetPathExists, err := targetPath.ExistCheck()
 			if err != nil {
-				return fmt.Errorf("Problem processing PROJECT_PATH argument value %v: %v", targetPath, err)
+				return fmt.Errorf("Unable to process PROJECT_PATH argument value %v: %v", targetPath, err)
 			}
 			if !targetPathExists {
 				return fmt.Errorf("PROJECT_PATH argument %v does not exist", targetPath)

Diff for: project/project.go

@@ -88,7 +88,7 @@ func findProjects(targetPath *paths.Path) ([]Type, error) {
 			return foundProjects, nil
 		}
 
-		return nil, fmt.Errorf("specified path %s is not an Arduino project", targetPath)
+		return nil, fmt.Errorf("Specified path %s is not an Arduino project", targetPath)
 	}
 
 	if configuration.SuperprojectTypeFilter() == projecttype.All || configuration.Recursive() {
@@ -110,7 +110,7 @@ func findProjects(targetPath *paths.Path) ([]Type, error) {
 	}
 
 	if foundProjects == nil {
-		return nil, fmt.Errorf("no projects found under %s", targetPath)
+		return nil, fmt.Errorf("No projects found under %s", targetPath)
 	}
 
 	return foundProjects, nil

Diff for: result/feedback/feedback.go

@@ -66,8 +66,9 @@ func Errorf(format string, v ...interface{}) {
 	Error(fmt.Sprintf(format, v...))
 }
 
-// Error behaves like fmt.Print but adds a newline and also logs the error.
+// Error behaves like fmt.Print but adds a prefix, newline and also logs the error.
 func Error(v ...interface{}) {
+	fmt.Fprint(os.Stderr, "error: ")
 	fmt.Fprintln(os.Stderr, v...)
 	logrus.Error(fmt.Sprint(v...))
 }

Diff for: result/result.go

@@ -236,13 +236,13 @@ func (results Type) WriteReport() error {
 	if !reportFilePathParentExists {
 		err = reportFilePath.Parent().MkdirAll()
 		if err != nil {
-			return fmt.Errorf("Error while creating report file path (%v): %v", reportFilePath.Parent(), err)
+			return fmt.Errorf("Unable to create report file path (%v): %v", reportFilePath.Parent(), err)
 		}
 	}
 
 	err = reportFilePath.WriteFile(results.jsonReportRaw())
 	if err != nil {
-		return fmt.Errorf("Error while writing report: %v", err)
+		return fmt.Errorf("While writing report: %v", err)
 	}
 
 	return nil