wxGUI/dbmgr: Show proper error when table parsing failed (#1112)

When parsing of a row fails, instead of trying to show error message
(and getting the GUI in a partially frozen state), raise exception
which the caller code expects and shows the message in a window
and allows user to continue working (although no table content is displayed).

The message also contains more details and suggests most likely cause (and fix).

Explicitly return None to make the intention clear.

This is a quick-fix. The real fix would be to use real CSV or JSON format and
parser as noted in the comment.

gui/wxpython/dbmgr/base.py

@@ -287,12 +287,24 @@ class VirtualAttributeList(ListCtrl,
 
             record = record.split(fs)
             if len(columns) != len(record):
-                GError(parent=self,
-                       message=_("Inconsistent number of columns "
-                                 "in the table <%(table)s>.") %
-                       {'table': tableName})
+                # Assuming there will be always at least one.
+                last = record[-1]
+                show_max = 3
+                if len(record) > show_max:
+                    record = record[:show_max]
+                # TODO: The real fix here is to use JSON output from v.db.select or
+                # proper CSV output and real CSV reader here (Python csv and json packages).
+                raise GException(
+                    _(
+                        "Unable to read the table <{table}> from the database due"
+                        " to seemingly inconsistent number of columns in the data transfer."
+                        " Check row: {row}..."
+                        " Likely, a newline character is present in the attribute value starting with: '{value}'"
+                        " Use the v.db.select module to investigate."
+                    ).format(table=tableName, row=" | ".join(record), value=last)
+                )
                 self.columns = {}  # because of IsEmpty method
-                return
+                return None
 
             self.AddDataRow(i, record, columns, keyId)