Add possiblity to check an error (exit code) of a postgres process command

[oilid]

Currently it's not possible to check an error (exit code) of a postgres process command. The problem ist the code in PostgresProcess.runCmd returns a String returned by "logWatch.getOutput()". But the best option would be to return the value of "proc.waitFor()" which give us the exit code of the process. By convention, 0 indicates normal termination.
Im my concrete case I needed this exit code for the method PostgresProcess.restoreFromFile to know if a database import was successful.
So in my opinion at least the following methods:

• PostgresProcess.importFromFile
• PostgresProcess.importFromFileWithArgs
• PostgresProcess.restoreFromFile
• PostgresProcess.exportToFile
• PostgresProcess.exportSchemeToFile
• PostgresProcess.exportDataToFile

should return the exitCode of the changed method "PostgresProcess.runCmd".

I just realized this issue in my fork (backport java 1.6): oilid/postgresql-embedded@22e649d