Report OutOfMemoryError when importing and exporting messages

When importing/exporting large files we can run out of memory, in
which case we should display a toast to the user (as we do for other
exceptions). Previously, the OutOfMemoryError was not caught because
it inherits from Error, not from Exception.

(We have to manually call .toString() because Context.showErrorToast()
from Commons takes an Exception or a String, not Throwable.)

app/src/main/kotlin/org/fossify/messages/activities/SettingsActivity.kt

@@ -134,8 +134,8 @@ class SettingsActivity : SimpleActivity() {
                     }
                     toast(org.fossify.commons.R.string.exporting_successful)
                 }
-            } catch (e: Exception) {
-                showErrorToast(e)
+            } catch (e: Throwable) { // also catch OutOfMemoryError etc.
+                showErrorToast(e.toString())
             }
         }
     }

app/src/main/kotlin/org/fossify/messages/helpers/MessagesImporter.kt

@@ -32,8 +32,8 @@ class MessagesImporter(private val activity: SimpleActivity) {
             } else {
                 importJson(uri)
             }
-        } catch (e: Exception) {
-            activity.showErrorToast(e)
+        } catch (e: Throwable) { // also catch OutOfMemoryError etc.
+            activity.showErrorToast(e.toString())
         }
     }