[gdb/testsuite] Fix error handling in _dap_read_json

In _dap_read_json we have a gdb_expect with clauses that generate errors:
...
        timeout {
            error "timeout reading json header"
        }
        eof {
            error "eof reading json header"
        }
...

Proc gdb_expect uses dejagnu's remote_expect, which has some peculiar
semantics related to errors:
...
 # remote_expect works basically the same as standard expect, but it
 # also takes care of getting the file descriptor from the specified
 # host and also calling the timeout/eof/default section if there is an
 # error on the expect call.
.....

When a timeout triggers, it generates a timeout error, which is reported by
gdb_expect, after which it runs the timeout/eof/default clauses, which
generates an eof error, which is reported by runtest.

I think the intention here is to generate just a one error, a timeout error.

Fix this by postponing generating the error until after gdb_expect.

Tested on x86_64-linux, by:
- running all the DAP test-cases and observing no regressions, and
- modifying the gdb.dap/eof.exp test-case to trigger a timeout error, and
  observing only a timeout error.

PR testsuite/31382
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31382

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