[lldb] Fix that log enable's -f parameter causes LLDB to crash when it can't open the log file

We didn't do anything with the llvm::Error we get from `Open`, so when we end up in the
error case we just crash due to the llvm::Error sanity check. Also add the missing newline
behind the error message so it no longer messes with the next (lldb) prompt.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D85970

lldb/source/Core/Debugger.cpp

@@ -1164,11 +1164,11 @@ bool Debugger::EnableLog(llvm::StringRef channel,
         flags |= File::eOpenOptionAppend;
       else
         flags |= File::eOpenOptionTruncate;
-      auto file = FileSystem::Instance().Open(
+      llvm::Expected<FileUP> file = FileSystem::Instance().Open(
           FileSpec(log_file), flags, lldb::eFilePermissionsFileDefault, false);
       if (!file) {
-        // FIXME: This gets garbled when called from the log command.
-        error_stream << "Unable to open log file: " << log_file;
+        error_stream << "Unable to open log file '" << log_file
+                     << "': " << llvm::toString(file.takeError()) << "\n";
         return false;
       }
 

lldb/test/API/commands/log/invalid-args/TestInvalidArgsLog.py

@@ -15,3 +15,8 @@ class InvalidArgsLogTestCase(TestBase):
     def test_disable_empty(self):
         self.expect("log disable", error=True,
                     substrs=["error: log disable takes a log channel and one or more log types."])
+
+    @no_debug_info_test
+    def test_enable_empty(self):
+        self.expect("log enable lldb all -f this/is/not/a/valid/path", error=True,
+                    substrs=["Unable to open log file 'this/is/not/a/valid/path': No such file or directory\n"])