Improve missing workflow err message [ci skip] (#2871)

Signed-off-by: Paolo Di Tommaso <paolo.ditommaso@gmail.com>
Co-authored-by: Phil Ewels <phil.ewels@scilifelab.se>

modules/nextflow/src/main/groovy/nextflow/script/BaseScript.groovy

@@ -184,8 +184,22 @@ abstract class BaseScript extends Script implements ExecutionContext {
         if( !entryFlow ) {
             if( meta.getLocalWorkflowNames() )
                 log.warn "No entry workflow specified"
-            if( meta.getLocalProcessNames() )
-                throw new AbortOperationException("Missing workflow definition - DSL2 requires at least a workflow block in the main script")
+            if( meta.getLocalProcessNames() ) {
+                final msg = """\
+                        =============================================================================
+                        =                                WARNING                                    =
+                        = You are running this script using DSL2 syntax, however it does not        = 
+                        = contain any 'workflow' definition so there's nothing for Nextflow to run. =
+                        =                                                                           =
+                        = If this script was written using Nextflow DSL1 syntax, please add the     = 
+                        = setting 'nextflow.enable.dsl=1' to the nextflow.config file or use the    =
+                        = command-line option '-dsl1' when running the pipeline.                    =
+                        =                                                                           =
+                        = More details at this link: https://www.nextflow.io/docs/latest/dsl2.html  =
+                        =============================================================================
+                        """.stripIndent()
+                throw new AbortOperationException(msg)
+            }
             return result
         }