show an error if the main file cannot be found

also improve logic for detecting the main file, if there's only one file
it must be the main file.

services/web/app/coffee/Features/Compile/ClsiManager.coffee

@@ -29,7 +29,11 @@ module.exports = ClsiManager =
 
 	sendRequestOnce: (project_id, user_id, options = {}, callback = (error, status, outputFiles, clsiServerId, validationProblems) ->) ->
 		ClsiManager._buildRequest project_id, options, (error, req) ->
-			return callback(error) if error?
+			if error?
+				if error.message is "no main file specified"
+					return callback(null, "validation-problems", null, null, {mainFile:error.message})
+				else
+					return callback(error)
 			logger.log project_id: project_id, "sending compile to CLSI"
 			ClsiFormatChecker.checkRecoursesForProblems req.compile?.resources, (err, validationProblems)->
 				if err?
@@ -206,9 +210,12 @@ module.exports = ClsiManager =
 		resources = []
 		rootResourcePath = null
 		rootResourcePathOverride = null
+		hasMainFile = false
+		numberOfDocsInProject = 0
 
 		for path, doc of docs
 			path = path.replace(/^\//, "") # Remove leading /
+			numberOfDocsInProject++
 			if doc.lines? # add doc to resources unless it is just a stub entry
 				resources.push
 					path:    path
@@ -217,11 +224,20 @@ module.exports = ClsiManager =
 				rootResourcePath = path
 			if options.rootDoc_id? and doc._id.toString() == options.rootDoc_id.toString()
 				rootResourcePathOverride = path
+			if path is "main.tex"
+				hasMainFile = true
 
 		rootResourcePath = rootResourcePathOverride if rootResourcePathOverride?
 		if !rootResourcePath?
-			logger.warn {project_id}, "no root document found, setting to main.tex"
-			rootResourcePath = "main.tex"
+			if hasMainFile
+				logger.warn {project_id}, "no root document found, setting to main.tex"
+				rootResourcePath = "main.tex"
+			else if numberOfDocsInProject is 1 # only one file, must be the main document
+				for path, doc of docs
+					rootResourcePath = path.replace(/^\//, "") # Remove leading /
+				logger.warn {project_id, rootResourcePath: rootResourcePath}, "no root document found, single document in project"
+			else
+				return callback new Error("no main file specified")
 
 		for path, file of files
 			path = path.replace(/^\//, "") # Remove leading /

services/web/app/views/project/editor/pdf.pug

@@ -317,6 +317,9 @@ div.full-size.pdf(ng-controller="PdfController")
 				div
 					li(ng-repeat="entry in pdf.validation.conflictedPaths") {{ '/'+entry['path'] }}
 
+			.alert.alert-danger(ng-show="pdf.validation.mainFile")
+				strong #{translate("main_file_not_found")} 
+				span #{translate("please_set_main_file")}
 
 		.pdf-errors(ng-switch-when="errors")