Change default inspection level to ERROR

This was briefly discussed in #77 (comment) but reverted from that MR.

I have found that separating mypy inspections from PyCharm's own type checker is helpful, because PyCharm is more prone to false positives.

It's also familiar to users of IDEA TypeScript integration.

src/main/resources/META-INF/plugin.xml

@@ -54,7 +54,7 @@
                          key="inspection.display-name"
                          groupKey="inspection.group"
                          shortName="Mypy"
-                         level="WARNING"
+                         level="ERROR"
                          unfair="true"
                          enabledByDefault="true"/>