don't duplicate explain error messages in explain

[eug3nix]

Pgmanage is showing "syntax error at or near..." error both in toast and in PEV2 error container when trying to run explain against an incorrect SQL.

Steps to reproduce:
0. Connect to Postgres DB

1. Enter an incorrect query like select 1=a
2. Run Explain

Observed:
Toast message with the error is shown
PEV2 also shows the same error

Expected:
Toast message is not shown when some other component displayed the error

Generally, errors in toast messages should be our last resort if the error was not handled interpreted and displayed by anyone else

[eug3nix]

fixed.