Actually return when reading file failed

Fixes #8258.

src/reader.cpp

@@ -4227,6 +4227,7 @@ static int read_ni(parser_t &parser, int fd, const io_chain_t &io) {
                 FLOGF(error, _(L"Unable to read input file: %s"), strerror(err));
                 // Reset buffer on error. We won't evaluate incomplete files.
                 fd_contents.clear();
+                return 1;
             }
         }
     }

tests/checks/invocation.fish

@@ -90,3 +90,7 @@ $fish --no-config -c 'echo $$ oh no syntax error' -c 'echo this works'
 # CHECKERR: fish: $$ is not the pid. In fish, please use $fish_pid.
 # CHECKERR: echo $$ oh no syntax error
 # CHECKERR: ^
+
+$fish --no-config .
+# CHECKERR: error: Unable to read input file: Is a directory
+# CHECKERR: warning: Error while reading file .