[libcalamares] Provide accessor to error code.

- Document meaning of error codes.
 - The test-loader considers internal errors a real (test) failure,
   while errors returned normally by the modules (e.g. because the
   configuration is broken) to be ok for testing purposes.

src/calamares/testmain.cpp

@@ -227,7 +227,8 @@ main( int argc, char* argv[] )
             cError() << "Job #" << count << "failed"
                 << TR( "summary", r.message() )
                 << TR( "details", r.details() );
-            ++failure_count;
+            if ( r.errorCode() > 0 )
+                ++failure_count;
         }
         ++count;
     }

src/libcalamares/Job.cpp

@@ -71,7 +71,7 @@ JobResult::ok()
 JobResult
 JobResult::error( const QString& message, const QString& details )
 {
-    return JobResult( message, details, -1 );
+    return JobResult( message, details, GenericError );
 }
 
 JobResult

src/libcalamares/Job.h

@@ -29,6 +29,13 @@ namespace Calamares {
 class DLLEXPORT JobResult
 {
 public:
+    /** @brief Distinguish classes of errors
+     *
+     * All "ok result" have errorCode 0 (NoError).
+     * Errors returned from job execution have values < 0.
+     * Errors before job execution, or not returned by the job execution
+     * itself, have values > 0.
+     */
     enum
     {
         NoError = 0,
@@ -49,6 +56,8 @@ public:
     virtual QString details() const;
     virtual void setDetails( const QString& details );
 
+    int errorCode() const { return m_number; }
+
     /// @brief an "ok status" result
     static JobResult ok();
     /// @brief an "error" result resulting from the execution of the job