Ticket #8625: sqlite-get_supported_schemes

Better reporting of installation issues related to PySqlite.

Use the same mechanism as for errors on VCS connectors (r7435).

Related to #8562.

diff --git a/trac/db/api.py b/trac/db/api.py
--- a/trac/db/api.py
+++ b/trac/db/api.py
@@ -37,6 +37,10 @@
     def get_supported_schemes():
         """Return the connection URL schemes supported by the connector, and
         their relative priorities as an iterable of `(scheme, priority)` tuples.
+        
+        If `priority` is a negative number, this is indicative of an
+        error  condition with the connector. An error message should be 
+        attached to the `error` attribute of the connector.
         """
 
     def get_connection(path, log=None, **kwargs):
@@ -121,6 +125,8 @@
             for scheme_, priority in connector.get_supported_schemes():
                 if scheme_ != scheme:
                     continue
+                if priority < 0:
+                    raise TracError(connector.error)
                 highest = candidates.get(scheme_, (None, 0))[1]
                 if priority > highest:
                     candidates[scheme] = (connector, priority)
diff --git a/trac/db/sqlite_backend.py b/trac/db/sqlite_backend.py
--- a/trac/db/sqlite_backend.py
+++ b/trac/db/sqlite_backend.py
@@ -22,6 +22,7 @@
 from trac.db.api import IDatabaseConnector
 from trac.db.util import ConnectionWrapper
 from trac.util import get_pkginfo, getuser
+from trac.util.translation import _
 
 _like_escape_re = re.compile(r'([/_%])')
 
@@ -112,9 +113,14 @@
 
     def __init__(self):
         self._version = None
+        self.error = None
 
     def get_supported_schemes(self):
-        return [('sqlite', 1)]
+        if have_pysqlite:
+            yield ('sqlite', 1)
+        else:
+            self.error = _("Coudn't load Python bindings for SQLite")
+            yield ('sqlite', -1)
 
     def get_connection(self, path, log=None, params={}):
         if not self._version: