Add dedicated error for "proc macro not found"

crates/hir_expand/src/proc_macro.rs

@@ -50,7 +50,7 @@ pub fn expand(
 
                 proc_macro.expander.expand(&tt, None).map_err(mbe::ExpandError::from)
             }
-            None => Err(err!("Unresolved proc macro")),
+            None => Err(mbe::ExpandError::UnresolvedProcMacro),
         }
     }
 }

crates/mbe/src/lib.rs

@@ -35,6 +35,7 @@ pub enum ExpandError {
     ConversionError,
     InvalidRepeat,
     ProcMacroError(tt::ExpansionError),
+    UnresolvedProcMacro,
     Other(String),
 }
 
@@ -53,6 +54,7 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
             ExpandError::ConversionError => f.write_str("could not convert tokens"),
             ExpandError::InvalidRepeat => f.write_str("invalid repeat expression"),
             ExpandError::ProcMacroError(e) => e.fmt(f),
+            ExpandError::UnresolvedProcMacro => f.write_str("unresolved proc macro"),
             ExpandError::Other(e) => f.write_str(e),
         }
     }