runtime: improve out-of-memory message when VirtualAlloc fails

Fixes #19514.

Change-Id: I93600d5c3d11ecab5a47dd4cd55ed3aea05e221e
Reviewed-on: https://go-review.googlesource.com/49611
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/mem_windows.go

@@ -16,6 +16,9 @@ const (
 
 	_PAGE_READWRITE = 0x0004
 	_PAGE_NOACCESS  = 0x0001
+
+	_ERROR_NOT_ENOUGH_MEMORY = 8
+	_ERROR_COMMITMENT_LIMIT  = 1455
 )
 
 // Don't split the stack as this function may be invoked without a valid G,
@@ -112,7 +115,13 @@ func sysMap(v unsafe.Pointer, n uintptr, reserved bool, sysStat *uint64) {
 	mSysStatInc(sysStat, n)
 	p := stdcall4(_VirtualAlloc, uintptr(v), n, _MEM_COMMIT, _PAGE_READWRITE)
 	if p != uintptr(v) {
-		print("runtime: VirtualAlloc of ", n, " bytes failed with errno=", getlasterror(), "\n")
-		throw("runtime: cannot map pages in arena address space")
+		errno := getlasterror()
+		print("runtime: VirtualAlloc of ", n, " bytes failed with errno=", errno, "\n")
+		switch errno {
+		case _ERROR_NOT_ENOUGH_MEMORY, _ERROR_COMMITMENT_LIMIT:
+			throw("out of memory")
+		default:
+			throw("runtime: cannot map pages in arena address space")
+		}
 	}
 }