More robust CpsFlowExecution.croak

[jglick]

The handling for a broken build in #809 seems to have been broken in the situation which #812 failed to reproduce:

org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
Script3: 8: unable to resolve class XXX 
 @ line 8, column 5.
       XXX xxx = null
       ^

Script3: 10: unable to resolve class XXX 
 @ line 10, column 20.
       void xxx(XXX xxx) {
                ^

2 errors

	at org.codehaus.groovy.control.ErrorCollector.failIfErrors(ErrorCollector.java:309)
	at org.codehaus.groovy.control.CompilationUnit.applyToSourceUnits(CompilationUnit.java:981)
	at org.codehaus.groovy.control.CompilationUnit.doPhaseOperation(CompilationUnit.java:626)
	at org.codehaus.groovy.control.CompilationUnit.compile(CompilationUnit.java:575)
	at groovy.lang.GroovyClassLoader.doParseClass(GroovyClassLoader.java:323)
	at groovy.lang.GroovyClassLoader.parseClass(GroovyClassLoader.java:293)
	at org.jenkinsci.plugins.scriptsecurity.sandbox.groovy.GroovySandbox$Scope.parse(GroovySandbox.java:163)
	at org.jenkinsci.plugins.workflow.cps.CpsGroovyShell.doParse(CpsGroovyShell.java:190)
	at org.jenkinsci.plugins.workflow.cps.CpsGroovyShell.reparse(CpsGroovyShell.java:175)
	at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.parseScript(CpsFlowExecution.java:638)
	at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.loadProgramAsync(CpsFlowExecution.java:855)
	at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.onLoad(CpsFlowExecution.java:825)
	at org.jenkinsci.plugins.workflow.job.WorkflowRun.getExecution(WorkflowRun.java:719)
	at org.jenkinsci.plugins.workflow.job.WorkflowRun.onLoad(WorkflowRun.java:577)
	at …
org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed: …
	at org.codehaus.groovy.control.ErrorCollector.failIfErrors(ErrorCollector.java:309)
	at org.codehaus.groovy.control.CompilationUnit.applyToSourceUnits(CompilationUnit.java:981)
	at org.codehaus.groovy.control.CompilationUnit.doPhaseOperation(CompilationUnit.java:626)
	at org.codehaus.groovy.control.CompilationUnit.compile(CompilationUnit.java:575)
	at groovy.lang.GroovyClassLoader.doParseClass(GroovyClassLoader.java:323)
	at …
java.util.EmptyStackException
	at java.base/java.util.Stack.peek(Stack.java:102)
	at java.base/java.util.Stack.pop(Stack.java:84)
	at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.onProgramEnd(CpsFlowExecution.java:1325)
	at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.croak(CpsFlowExecution.java:914)
	at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.loadProgramFailed(CpsFlowExecution.java:908)
	at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.loadProgramAsync(CpsFlowExecution.java:893)
	at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.onLoad(CpsFlowExecution.java:825)
	at org.jenkinsci.plugins.workflow.job.WorkflowRun.getExecution(WorkflowRun.java:719)
	at org.jenkinsci.plugins.workflow.job.WorkflowRun.onLoad(WorkflowRun.java:577)
	at …
ERROR: Failed to load program
Finished: FAILURE