core: fix description of "resources" service error (#3119)

The "resources" error is really just the generic error we return when
we hit some kind of error and we have no more appropriate error for the case to
return, for example because of some OS error.

Hence, reword the explanation and don't claim any relation to resource limits.

Admittedly, the "resources" service error is a bit of a misnomer, but I figure
it's kind of API now.

Fixes: #2716

src/core/service.h

@@ -80,7 +80,7 @@ typedef enum NotifyState {
 
 typedef enum ServiceResult {
         SERVICE_SUCCESS,
-        SERVICE_FAILURE_RESOURCES,
+        SERVICE_FAILURE_RESOURCES, /* a bit of a misnomer, just our catch-all error for errnos we didn't expect */
         SERVICE_FAILURE_TIMEOUT,
         SERVICE_FAILURE_EXIT_CODE,
         SERVICE_FAILURE_SIGNAL,

src/shared/bus-unit-util.c

@@ -684,7 +684,7 @@ static int bus_job_get_service_result(BusWaitForJobs *d, char **result) {
 static const struct {
         const char *result, *explanation;
 } explanations [] = {
-        { "resources",   "a configured resource limit was exceeded" },
+        { "resources",   "of unavailable resources or another system error" },
         { "timeout",     "a timeout was exceeded" },
         { "exit-code",   "the control process exited with error code" },
         { "signal",      "a fatal signal was delivered to the control process" },