Clarify error messages on corefiles that no plugin handles (#72559)

These error messages are written in a way that makes sense to an lldb
developer, but not to an end user who asks lldb to run on a compressed
corefile or whatever. Simplfy the messages.

Author: jasonmolenda

lldb/source/Commands/CommandObjectTarget.cpp

@@ -436,8 +436,7 @@ class CommandObjectTargetCreate : public CommandObjectParsed {
           error = process_sp->LoadCore();
 
           if (error.Fail()) {
-            result.AppendError(
-                error.AsCString("can't find plug-in for core file"));
+            result.AppendError(error.AsCString("unknown core file format"));
             return;
           } else {
             result.AppendMessageWithFormatv(
@@ -447,9 +446,8 @@ class CommandObjectTargetCreate : public CommandObjectParsed {
             on_error.release();
           }
         } else {
-          result.AppendErrorWithFormatv(
-              "Unable to find process plug-in for core file '{0}'\n",
-              core_file.GetPath());
+          result.AppendErrorWithFormatv("Unknown core file format '{0}'\n",
+                                        core_file.GetPath());
         }
       } else {
         result.AppendMessageWithFormat(

lldb/source/Core/IOHandlerCursesGUI.cpp

@@ -3178,13 +3178,13 @@ class TargetCreateFormDelegate : public FormDelegate {
         m_debugger.GetListener(), llvm::StringRef(), &core_file_spec, false));
 
     if (!process_sp) {
-      SetError("Unable to find process plug-in for core file!");
+      SetError("Unknown core file format!");
       return;
     }
 
     Status status = process_sp->LoadCore();
     if (status.Fail()) {
-      SetError("Can't find plug-in for core file!");
+      SetError("Unknown core file format!");
       return;
     }
   }

lldb/test/API/commands/target/basic/TestTargetCommand.py

@@ -447,7 +447,7 @@ def test_target_create_invalid_core_file(self):
         self.expect(
             "target create -c '" + invalid_core_path + "'",
             error=True,
-            substrs=["Unable to find process plug-in for core file '"],
+            substrs=["Unknown core file format '"],
         )
 
     # Write only files don't seem to be supported on Windows.