improved error message in case of non existing configuration

src/main/kotlin/net/woggioni/plugins/DependencyExportPlugin.kt

@@ -58,9 +58,18 @@ object DependencyExporter {
         var sequence = 0
         val map = HashMap<ResolvedComponentResult, Int>()
 
-        val resolutionResult = project.configurations.single {
+        val requestedConfiguration = project.configurations.singleOrNull {
             it.name == ext.configurationName
-        }.incoming.resolutionResult
+        }?.takeIf { it.isCanBeResolved } ?: let {
+            val resolvableConfigurations = "[" + project.configurations.asSequence()
+                    .filter { it.isCanBeResolved }
+                    .map { "'${it.name}'" }
+                    .joinToString(",") + "]"
+            throw GradleException("Configuration '${ext.configurationName}' doesn't exist or cannot be resolved, " +
+                "resolvable configurations in this project are " + resolvableConfigurations)
+        }
+
+        val resolutionResult = requestedConfiguration.incoming.resolutionResult
         if (!ext.outputFile.isAbsolute) {
             ext.outputFile = project.buildDir.toPath().resolve(ext.outputFile)
         }