External merge tools: better error message for unsupported usage

I've heard of one instance of a person being confused by the error.

Previously, the error was:

```
Error: Failed to load tool configuration
Caused by: To use `diffedit3` as a merge tool, the config `merge-tools.diffedit3.merge-args` must be defined (see docs for details)

```

Now, it is:

```
Error: The tool `diffedit3` cannot be used as a merge tool with `jj resolve`.
Hint: To use `diffedit3` as a merge tool, the config `merge-tools.diffedit3.merge-args` must be defined (see docs for details)
```

TODO for future PR: allow setting `merge-tools.TOOL.edit-args = false` so that
attempting to use TOOL as a diff editor fails. This would be helpful, for
example, for the `vscode` tool.

cli/src/command_error.rs

@@ -375,7 +375,20 @@ impl From<ConflictResolveError> for CommandError {
 
 impl From<MergeToolConfigError> for CommandError {
     fn from(err: MergeToolConfigError) -> Self {
-        user_error_with_message("Failed to load tool configuration", err)
+        match err {
+            MergeToolConfigError::MergeArgsNotConfigured { ref tool_name } => {
+                let tool_name = tool_name.clone();
+                user_error_with_hint(
+                    err,
+                    format!(
+                        "To use `{tool_name}` as a merge tool, the config \
+                         `merge-tools.{tool_name}.merge-args` must be defined (see docs for \
+                         details)"
+                    ),
+                )
+            }
+            _ => user_error_with_message("Failed to load tool configuration", err),
+        }
     }
 }
 

cli/src/merge_tools/mod.rs

@@ -91,10 +91,7 @@ pub enum ConflictResolveError {
 pub enum MergeToolConfigError {
     #[error(transparent)]
     Config(#[from] ConfigError),
-    #[error(
-        "To use `{tool_name}` as a merge tool, the config `merge-tools.{tool_name}.merge-args` \
-         must be defined (see docs for details)"
-    )]
+    #[error("The tool `{tool_name}` cannot be used as a merge tool with `jj resolve`.")]
     MergeArgsNotConfigured { tool_name: String },
 }