Rename linux.Errno.Error to linux.Errno.String.

Using linux.Errno as an error doesn't work very well as none of the sentry code
expects error to contain a linux.Errno.

This moves using syserr.Error.ToLinux as an error in a syscall handler from a
runtime error to a compile error.

PiperOrigin-RevId: 227744312
Change-Id: Iea63108a5b198296c908614e09c01733dd684da0

pkg/abi/linux/errors.go

@@ -25,8 +25,8 @@ func (e *Errno) Number() int {
 	return e.number
 }
 
-// Error implements error.Error.
-func (e *Errno) Error() string {
+// String implements fmt.Stringer.String.
+func (e *Errno) String() string {
 	return e.name
 }