Improve dev-certs export error message

During a recent security review of the dev-certs tool, we observed that on export it would create a directory that was potentially world-readable (e.g. based on permissions inherited from the parent directory).  We decided it would be more appropriate to let users make the decision of who should have access to the directory.  Unfortunately, this removal of functionality broke some app authors' workflows.  When dev-certs is run directly, the `--verbose` output makes it clear what went wrong and what needs to happen, but the non-verbose output that appears when another tool does the export is less helpful.  This change introduces a new top-level error state for an export failure caused by a non-existent target directory to make it clearer how to fix broken workflows.

The behavior changed in dotnet#57108, which included a backport of dotnet#56985, and shipped in 8.0.10.

For dotnet#58330

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.");
+                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.");