[processing][grass] Highlight GRASS error messages and warnings in log

And catch segmentation faults from GRASS commands and show a nice
helper text advising user that QGIS isn't at fault here ;)

Also add a tip to try disabling the v.external option if that setting
is enabled and a GRASS segfault occurs

(cherry picked from commit 676238b)

python/plugins/processing/algs/grass7/Grass7Utils.py

@@ -379,7 +379,16 @@ def executeGrass(commands, feedback, outputCommands=None):
                     if 'r.out' in line or 'v.out' in line:
                         grassOutDone = True
                     loglines.append(line)
-                    feedback.pushConsoleInfo(line)
+                    if any([l in line for l in ['WARNING', 'ERROR']]):
+                        feedback.reportError(line.strip())
+                    elif 'Segmentation fault' in line:
+                        feedback.reportError(line.strip())
+                        feedback.reportError('\n' + Grass7Utils.tr('GRASS command crashed :( Try a different set of input parameters and consult the GRASS algorithm manual for more information.') + '\n')
+                        if ProcessingConfig.getSetting(Grass7Utils.GRASS_USE_VEXTERNAL):
+                            feedback.reportError(Grass7Utils.tr(
+                                'Suggest disabling the experimental "use v.external" option from the Processing GRASS Provider options.') + '\n')
+                    elif line.strip():
+                        feedback.pushConsoleInfo(line.strip())
 
         # Some GRASS scripts, like r.mapcalculator or r.fillnulls, call
         # other GRASS scripts during execution. This may override any
@@ -405,9 +414,12 @@ def executeGrass(commands, feedback, outputCommands=None):
                                 line[len('GRASS_INFO_PERCENT') + 2:]))
                         except:
                             pass
-                    else:
-                        loglines.append(line)
-                        feedback.pushConsoleInfo(line)
+                    if any([l in line for l in ['WARNING', 'ERROR']]):
+                        loglines.append(line.strip())
+                        feedback.reportError(line.strip())
+                    elif line.strip():
+                        loglines.append(line.strip())
+                        feedback.pushConsoleInfo(line.strip())
 
         if ProcessingConfig.getSetting(Grass7Utils.GRASS_LOG_CONSOLE):
             QgsMessageLog.logMessage('\n'.join(loglines), 'Processing', Qgis.Info)