Better error handling for pipeline rule failures

## Expected Behavior
The server should feedback that there was a failure to save the rule (with a meaningful error message on why).

## Current Behavior
Fatals in the log but no feedback to the user.

```
2021-02-25T01:26:08.902Z ERROR [AnyExceptionClassMapper] Unhandled exception in REST resource
java.lang.NullPointerException: null
at org.graylog.plugins.pipelineprocessor.parser.PipelineRuleParser$RuleTypeChecker.exitMessageRef(PipelineRuleParser.java:842) ~[graylog.jar:?]
at org.graylog.plugins.pipelineprocessor.parser.RuleLangParser$MessageRefContext.exitRule(RuleLangParser.java:729) ~[graylog.jar:?]
at org.antlr.v4.runtime.tree.ParseTreeWalker.exitRule(ParseTreeWalker.java:47) ~[graylog.jar:?]
at org.antlr.v4.runtime.tree.ParseTreeWalker.walk(ParseTreeWalker.java:30) ~[graylog.jar:?]
at org.antlr.v4.runtime.tree.ParseTreeWalker.walk(ParseTreeWalker.java:28) ~[graylog.jar:?]
at org.antlr.v4.runtime.tree.ParseTreeWalker.walk(ParseTreeWalker.java:28) ~[graylog.jar:?]
at org.antlr.v4.runtime.tree.ParseTreeWalker.walk(ParseTreeWalker.java:28) ~[graylog.jar:?]
at org.antlr.v4.runtime.tree.ParseTreeWalker.walk(ParseTreeWalker.java:28) ~[graylog.jar:?]
at org.graylog.plugins.pipelineprocessor.parser.PipelineRuleParser.parseRule(PipelineRuleParser.java:171) ~[graylog.jar:?]
at org.graylog.plugins.pipelineprocessor.parser.PipelineRuleParser.parseRule(PipelineRuleParser.java:134) ~[graylog.jar:?]
at org.graylog.plugins.pipelineprocessor.rest.RuleResource.parse(RuleResource.java:119) ~[graylog.jar:?]
at jdk.internal.reflect.GeneratedMethodAccessor439.invoke(Unknown Source) ~[?:?]
at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:?]
at java.lang.reflect.Method.invoke(Method.java:566) ~[?:?]
at org.glassfish.jersey.server.model.internal.ResourceMethodInvocationHandlerFactory.lambda$static$0(ResourceMethodInvocationHandlerFactory.java:52) ~[graylog.jar:?]
at org.glassfish.jersey.server.model.internal.AbstractJavaResourceMethodDispatcher$1.run(AbstractJavaResourceMethodDispatcher.java:124) ~[graylog.jar:?]
at org.glassfish.jersey.server.model.internal.AbstractJavaResourceMethodDispatcher.invoke(AbstractJavaResourceMethodDispatcher.java:167) ~[graylog.jar:?]
at org.glassfish.jersey.server.model.internal.JavaResourceMethodDispatcherProvider$TypeOutInvoker.doDispatch(JavaResourceMethodDispatcherProvider.java:219) ~[graylog.jar:?]
at org.glassfish.jersey.server.model.internal.AbstractJavaResourceMethodDispatcher.dispatch(AbstractJavaResourceMethodDispatcher.java:79) ~[graylog.jar:?]
at org.glassfish.jersey.server.model.ResourceMethodInvoker.invoke(ResourceMethodInvoker.java:469) ~[graylog.jar:?]
at org.glassfish.jersey.server.model.ResourceMethodInvoker.apply(ResourceMethodInvoker.java:391) ~[graylog.jar:?]
at org.glassfish.jersey.server.model.ResourceMethodInvoker.apply(ResourceMethodInvoker.java:80) ~[graylog.jar:?]
at org.glassfish.jersey.server.ServerRuntime$1.run(ServerRuntime.java:253) [graylog.jar:?]
at org.glassfish.jersey.internal.Errors$1.call(Errors.java:248) [graylog.jar:?]
at org.glassfish.jersey.internal.Errors$1.call(Errors.java:244) [graylog.jar:?]
at org.glassfish.jersey.internal.Errors.process(Errors.java:292) [graylog.jar:?]
at org.glassfish.jersey.internal.Errors.process(Errors.java:274) [graylog.jar:?]
at org.glassfish.jersey.internal.Errors.process(Errors.java:244) [graylog.jar:?]
at org.glassfish.jersey.process.internal.RequestScope.runInScope(RequestScope.java:265) [graylog.jar:?]
at org.glassfish.jersey.server.ServerRuntime.process(ServerRuntime.java:232) [graylog.jar:?]
at org.glassfish.jersey.server.ApplicationHandler.handle(ApplicationHandler.java:680) [graylog.jar:?]
at org.glassfish.jersey.grizzly2.httpserver.GrizzlyHttpContainer.service(GrizzlyHttpContainer.java:356) [graylog.jar:?]
at org.glassfish.grizzly.http.server.HttpHandler$1.run(HttpHandler.java:200) [graylog.jar:?]
at com.codahale.metrics.InstrumentedExecutorService$InstrumentedRunnable.run(InstrumentedExecutorService.java:180) [graylog.jar:?]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) [?:?]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) [?:?]
at java.lang.Thread.run(Thread.java:834) [?:?]
```

## Steps to Reproduce (for bugs)

1. Create a pipeline rule with this content

```
rule "syslog-ng"
when
has_field("mediawiki_@timestamp")
then
let new_date = parse_date( to_string($message."mediawiki_@timestamp"),"yyyy-MM-dd HH:mm:ss.SSSX","UTC");
set_field("timestamp", new_date);
debug($message);
end
```

## Context
I didn't know it was fataling and only knew as the error log showed it.

## Your Environment

* Graylog Version: 4.0.2-1
* Java Version: 11.0.9.1
* Elasticsearch Version: 7.10.0
* MongoDB Version: 4.4
* Operating System: Debian 10
* Browser version: 88.0.4324.192