create_buffer_from_file: throw file_not_found instead of open_file_error when ENOENT

src/file.cc

@@ -14,7 +14,12 @@ Buffer* create_buffer_from_file(const std::string& filename)
 {
     int fd = open(filename.c_str(), O_RDONLY);
     if (fd == -1)
+    {
+        if (errno == ENOENT)
+            throw file_not_found(strerror(errno));
+
         throw open_file_error(strerror(errno));
+    }
 
     std::string content;
     char buf[256];

src/file.hh

@@ -13,6 +13,12 @@ struct open_file_error : public std::runtime_error
         : std::runtime_error(what) {}
 };
 
+struct file_not_found : public open_file_error
+{
+    file_not_found(const std::string& what)
+        : open_file_error(what) {}
+};
+
 struct write_file_error : public std::runtime_error
 {
     write_file_error(const std::string& what)