[release/9.0] Improve dev-certs export error message

src/Shared/CertificateGeneration/CertificateManager.cs

@@ -328,6 +328,7 @@ public EnsureCertificateResult EnsureAspNetCoreHttpsDevelopmentCertificate(
                 var exportDir = Path.GetDirectoryName(path);
                 if (!string.IsNullOrEmpty(exportDir) && !Directory.Exists(exportDir))
                 {
+                    result = EnsureCertificateResult.ErrorExportingTheCertificateToNonExistentDirectory;
                     throw new InvalidOperationException($"The directory '{exportDir}' does not exist.  Choose permissions carefully when creating it.");
                 }
 

src/Shared/CertificateGeneration/EnsureCertificateResult.cs

@@ -10,6 +10,7 @@ internal enum EnsureCertificateResult
     ErrorCreatingTheCertificate,
     ErrorSavingTheCertificateIntoTheCurrentUserPersonalStore,
     ErrorExportingTheCertificate,
+    ErrorExportingTheCertificateToNonExistentDirectory,
     FailedToTrustTheCertificate,
     PartiallyFailedToTrustTheCertificate,
     UserCancelledTrustStep,

src/Tools/FirstRunCertGenerator/test/CertificateManagerTests.cs

@@ -373,7 +373,7 @@ public void EnsureCreateHttpsCertificate_CannotExportToNonExistentDirectory()
             .EnsureAspNetCoreHttpsDevelopmentCertificate(now, now.AddYears(1), Path.Combine("NoSuchDirectory", CertificateName));
 
         // Assert
-        Assert.Equal(EnsureCertificateResult.ErrorExportingTheCertificate, result);
+        Assert.Equal(EnsureCertificateResult.ErrorExportingTheCertificateToNonExistentDirectory, result);
     }
 
     [Fact]

src/Tools/dotnet-dev-certs/src/Program.cs

@@ -425,6 +425,10 @@ private static int EnsureHttpsCertificate(CommandOption exportPath, CommandOptio
             case EnsureCertificateResult.ErrorExportingTheCertificate:
                 reporter.Warn("There was an error exporting the HTTPS developer certificate to a file.");
                 return ErrorExportingTheCertificate;
+            case EnsureCertificateResult.ErrorExportingTheCertificateToNonExistentDirectory:
+                // A distinct warning is useful, but a distinct error code is probably not.
+                reporter.Warn("There was an error exporting the HTTPS developer certificate to a file. Please create the target directory before exporting. Choose permissions carefully when creating it.");
+                return ErrorExportingTheCertificate;
             case EnsureCertificateResult.PartiallyFailedToTrustTheCertificate:
                 // A distinct warning is useful, but a distinct error code is probably not.
                 reporter.Warn("There was an error trusting the HTTPS developer certificate. It will be trusted by some clients but not by others.");