Throw better error if lockfile is not writable.

src/files.cpp

@@ -229,8 +229,16 @@ bool create_lockfile()
 
     ofstream file;
     file.exceptions(std::ofstream::failbit | std::ofstream::badbit);
-    file.open(lockfile);
-    file << getpid();
+    try
+    {
+        file.open(lockfile);
+        file << getpid();
+    }
+    catch (const std::ofstream::failure &e)
+    {
+        const auto msg{string("Could not write lockfile: ") += e.what()};
+        throw std::runtime_error{msg.c_str()};
+    }
 
     return true;
 }