[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

gdb/testsuite/lib/dap-support.exp

@@ -147,6 +147,8 @@ proc dap_send_request {command {obj {}}} {
 # "last_ton" will be set to the TON form of the result.
 proc _dap_read_json {} {
     set length ""
+    set seen_timeout 0
+    set seen_eof 0
     gdb_expect {
 	-re "^Content-Length: (\[0-9\]+)\r\n" {
 	    set length $expect_out(1,string)
@@ -160,13 +162,20 @@ proc _dap_read_json {} {
 	    # Done.
 	}
 	timeout {
-	    error "timeout reading json header"
+	    set seen_timeout 1
 	}
 	eof {
-	    error "eof reading json header"
+	    set seen_eof 1
 	}
     }
 
+    if {$seen_timeout} {
+	error "timeout reading json header"	
+    }
+    if {$seen_eof} {
+	error "eof reading json header"
+    }
+
     if {$length == ""} {
 	error "didn't find content-length"
     }
@@ -176,17 +185,27 @@ proc _dap_read_json {} {
 	# Tcl only allows up to 255 characters in a {} expression in a
 	# regexp, so we may need to read in chunks.
 	set this_len [expr {min ($length, 255)}]
+	set seen_timeout 0
+	set seen_eof 0
 	gdb_expect {
 	    -re "^.{$this_len}" {
 		append json $expect_out(0,string)
 	    }
 	    timeout {
-		error "timeout reading json body"
+		set seen_timeout 1
 	    }
 	    eof {
-		error "eof reading json body"
+		set seen_eof 1
 	    }
 	}
+
+	if {$seen_timeout} {
+	    error "timeout reading json header"	
+	}
+	if {$seen_eof} {
+	    error "eof reading json header"
+	}
+
 	incr length -$this_len
     }